This is long, because I have had a great deal of fun exploring magic from an information theory perspective. There's a separator half way through for those who just want to read how Rynn should behave.
I would approach this from a key observation: others will observe the effects of Rynn's magic. If they don't, it makes for very poor magic. Something should happen. While it may seem like the secret to subtle magic is to make as "small" of a change as possible, a more precise wording is helpful: the secret to subtle magic is to do things in a way which is easily explained by The Unknown or to encourage others to not search for an explanation in the first place.
In science, The Unknown is modeled as random variables. However, The Unknown takes on many forms in other approaches to making sense of the world. The Norse might call it Loki's mischief. I've heard it called the devil's handiwork before. Whatever you call it, it represents that of the world which you did not measure, thus cannot predict its effects.
This approach is particularly convenient for modeling such subtle magic because it lends itself to an easy study using information theory. Assume we all have some information about the world. As we interact with it, we learn more. We also forget things which are of lower value (consider: the third letter of this paragraph is an 'i,' but you didn't think it was important enough to remember that, did you?)
To elicit a subtle magic effect is akin to being able to see the world in a different way. Consider the subtle magic of a teenager fixing their grandparent's computer. The way the grandparents view the world is valuable in many ways, but in the particular case of computers, their worldview is highly ineffective at solving problems. The teenager, having grown up with computers, can easily see the root causes of computer related problems and find solutions. From the grandparents' point of view, what the teenager does is indistinguishable from a subtle magic. They simply cannot see enough information to explain why the teenager's approach yields success when theirs fails. All they can do is keep his or her number on speed dial and thank the stars that they don't have to solve these problems on their own.
Such a world view can be viewed as a body of information itself. The worldview is made up of many assumptions and patterns that have been useful in the past for understanding the world with as little effort as possible. Being information, it can be shared. This is where our teenager's plight differs from Rynn's. While the teenager would certainly love it if their grandparents figured out how to work a computer, Rynn has a vested interest in not letting them do so. If everyone could see the world the way she does, then they could predict her abilities in advance and effectively nullify them. She needs to keep this worldview secret. This leads to what I would call the first rule of keeping magic:
Magic must not "leak" information about how it approaches the world, or it becomes commonplace, just as the magic of flight is now a daily commute for many.
So how do we avoid leaks. There are two fundamental techniques I can identify:
- Don't emit any information.
- "Whiten" the leaked information to make it appear more like The Unknown before emitting it.
- Gather information that others do not know, and obscure the leaked information with it.
The first solution is easy. If you don't emit any information about your worldview, you are safe. However, this is very difficult in the face of science. Science is very good at collecting multiple datapoints and mining them for data. The one escape: do magic only once. In many magic systems we see the concept of someone getting to do a "miracle," but often they can only do one. The idea is that each person has something that makes them "them." Nobody else has it. If you are willing to give it up, you can do tremendous magic. However, afterwards, everyone knows that little bit that makes you "you." With that information, they can identify how you did the magic, and it ceases to become magic. However, in the case of miracles, the effect is already done. It occurred too fast to prevent the first time; all the world can do is prevent it from happening again.
The next two solutions both involve "whitening" the information to make it harder to discern from The Unknown. This process is easily seen in modern computer cryptography. Two individuals with a shared secret can communicate using an encryption which others cannot penetrate (such as AES). One way this can be applied to magic is if a founder of a magic school can split the magic into two parts which functions similar to public key cryptography. The founder breaks the magic into a public and private part. The public part is the one which does all of the work of magic, but it can only do it with the help of the private part. The private part contains the secret keys to the art shared only between those in the school of magic and the source of the magic itself (perhaps the universe). When "casting a spell," the inner part allows for an interaction with the source of magic which appears to be noise unless you have the secret key. The source then provides you the power needed to complete the spell using the public part of the magic.
This pattern shows up in secret societies. When a magical group has secret rituals, they form the backbone of that inner "private" key. If you could observe those rituals, you could dismantle their power. They keep them secret. However, you see the outer "public" key, which is the powerful magics they wield (such as the ability to cause rain to fall).
Of course this has two fundamental ways to fall apart. The first is obvious: if the secret rituals are exposed, so is the root of their power (akin to Sampson of the Bible having his hair cut). The second is more subtle: your power is only protected by how effectively your secret rituals actually guard your abilities. Consider the ENIGMA, which had the magical ability to protect German U-Boat movements until mathematicians in England figured out exploits to uncover the secret keys. Secrets get broken all the time.
The final solution in my list is to acquire information which is not known by anyone else, and use it to "whiten" the magical information. This has a dark side and a light side. The dark side is visible in many magic systems: the ability to take information by force. Sacrifices and blood thaumaturgy are examples of using something which has never been exposed to anyone else and using that to whiten the magic.
Before going onto the light side, I'd point out the middle ground you will find between them: chance. If your magic works if a coin flip is "heads" and fails if the coin flip is "tails," then it leaks 50% as much information with each usage.
The light side is to use only information which is given freely. Secrets, promises, locks of hair: these are often given as "payment" for magic. These contain enough information to obscure the magic from the world, making it appear Unknown.
The lightest of the light side is to use only information which is forgotten or left behind. Most people forget how many steps they took from the cab to the front door, or whether they turned the doorknob clockwise or counterclockwise to enter. This is enough to "whiten" very strong magic, so long as nobody ever catches on to how you're doing it.
So let's get to Rynn. How does Rynn remain undetected. Of all of the methods of obscuring magic, the only one which is reliably undetectable is to collect that which is forgotten. However, we run into a bit of an issue: nothing is ever truly forgotten. Someone may forget their hat, only to remember it and come back later. If she were to rely on such forgotten things, she would eventually be trapped.
There is one pattern that is demonstrably undetectable. Many interactions are not fully observable. Push on someone and it's hard to tell if they're just really light for their size, or if they helped you by moving with you instead of resisting. Shake someone's hand, and it is hard to tell if you are enthusiastic to meet them, or if they are enthusiastic to meet you; both cause the handshake to pump up and down the same. In these situations, each party only observes at most half of the information in total. The other half is free to be used for whitening.
Now eventually someone will catch on. After all, you only get lucky so many times. Someone will eventually figure out what Rynn is doing if she's not careful. She needs a second layer of defense - one which your description captured perfectly. She needs people to want to believe she's just lucky. Accordingly, she needs to seek out win-win situations, where she benefits and the other party benefits. Hence the curious tendency for people to just get better around her. This would result in a tendency for people to begin to migrate towards her instinctively.
Eventually people do realize there's something special about her, but if they are comfortable enough with that level of specialness, they wont pry. (Interestingly enough, this is a strong in-character corollary to Sanderson's First Law of Magic, "The ability for an author to use magic to resolve conflict is directly proportional to how much the reader understands it.")
Now for actions. These are the interesting part. Consider that an ill-worded answer to a sharply phrased question could reveal a little of her magic. Too many such answers could box her in, forcing her to reveal more than she wants to. She would have a strong tendency to avoid giving answers to sharp questions - ideally by misdirecting away from them, but she would resort to vague answers if needed. Her actions would be similarly vague. If people are naturally attracted to her, she would need to be able to move agilely to avoid being smothered by them (physically and socially).
The rest of society can help. Attitudes such as "do what you will, may it harm none" would allow much more room for Rynn to do extraordinary things without bothering people. Scientific thought would be the most difficult attitude for her to cope with. Scientists would constantly be trying to fix the variables she needs to whiten her magic. One strong sign of this would be people asking for repeat performances of previous magic (to which she would never do exactly the same thing twice. Each magic would be independent to the circumstances). A fear of The Unknown could result in a violent confrontation with Rynn, for her power depends on the ability to blend in with The Unknown. If it, itself, is hated, then blending in is much less useful.