Let's imagine humans are somehow controlled by a smart telepathic species that make themselves invisible to them, say everyone has an octopi on their head and are unaware of it. Every time the cephalopod senses the host is becoming aware of it, the octopi steers the host away from that idea. How would the host detect the guest anyway, assuming it's not a life death situation, more like a game?
Bonus points for swapping places in a way that doesn't make her aware of it.
The simplest answer is that the octopus will be discovered in day-to-day life.
Lets say your human always sleeps late, but one night your human wakes up because he's got the flu and can't sleep, but the octopus in question is still asleep somewhere unable to mindcontrol himself away. That's going to be a discovery right there.
Lets say your human hits puberty and is goofing around, playing sports or similar. During this there is a fair chance the octopus gets hurt enough that he can't fulfill his obfuscation act anymore. Similarly during accidents the octopus can be discovered.
If the octopus gets sick he could also be discovered, worse if the octopus gets fever dreams and mentally lashes out at nearby humans. You could also have dumb octopi or octopi with disabilities, or even just "plain" depression that aren't able to keep the ruse up.
Then there's designing the world. If you put on a hood because it rains the octopus needs to obfuscate the fact that he's on your head and the hood doesn't fit. You could argue that the octopus could hide on the shoulder instead, but exposure to the elements is a solid way to die for such a creature. It doesn't matter if its heat, cold, sun exposure etc, if the octopus is more vulnerable he'll be disabled much quicker. A way around this is to design the hood to accomodate the octopus. Unfortunately you are now looking at the task of every octopus that witnesses anything being designed in the world to accomodate for a wide variety of situations for the octopus. How do you deploy an airbag in a car and still protect the octopus properly? How do you deal with the overload of opening a clothing closet filled with modified hoodies and the octopus having to erase every idea that they are modified hoodies? How do you make sure every roof of every vehicle and room is high enough to accomodate the octopus?
The top of my head was chopped off and I'm still moving.
Since everyone has an octopus on their head, the octopuses have agreed they will all have their host believe the extra wobblier top part of their head is completely natural and has always been there. This accounts for all physical ways of discovering the octopus. Of course my hood is extra large -- I have a huge head!
It also means anyone who starts to suspect the truth will be frowned upon by the others and regarded as crazy.
This works until the top of someone's head is chopped off in an accident. Strangely enough they are still alive and now old memories are returning. . . .
Competition.
I am a mind controlling octopus and I have recently levelled up. I am at the top of my game. And I am sick of riding around on Willk. He has some sort of intestinal problem and his knees are shot and it is getting tiresome. I want to ride around on @boring!
So I jump on. @Boring's current octopus has to fight for control but that featherweight chump has not got a chance against me. From my (temporary) position on @borings buttocks I plant thoughts in @boring encouraging the use of harsh hair products and the wearing of tight hats. I encourage @boring to take up soccer again and go for all the headers. Maybe I can even plant some ideas in the old octopus' head - like @boring has bad knees and some sort of intestinal problem... hmmm... nah.
The old octopus realizes it is outmatched and cedes control. As I ascend to @boring's head to take the reins, the previous resident moves onto @Demigan to try the tricks it just learned from me as well as some new additions it has devised involving heavy metal headbanging.
