Consider the following situation: Over the last 2 to 3 years, all the world's birds and bats outside of those in human captivity or domestication have stopped getting sick; sick - even terminally sick - animals have recovered, and these animals now have lifespans extended to about double that which they formerly could have expected.
Should humans capture any of these birds or bats, their immunity ceases and their aging progresses as usual from the creature's apparent age. Any birds or bats released or escaped from human captivity and observation also quickly (re)acquire this newfound immunity and longevity.
Should a captured bird or bat be examined sufficiently closely soon after capture, scientists might notice trace quantities of Hafnium, Tantalum, Silver, Holmium and Lutetium above those normally found in birds, as well as subtle differences in bone layering, but no other explanation for their former immunities and longevity.
The agency by which this is achieved is an alien nanotechnology with sophisticated rod logic brains that have been instructed to covertly spy upon humans, and if any humans get too close, the nanotech will self-destruct without harming the animal (leaving behind the trace elements I mentioned, plus Carbon, Nitrogen, Oxygen and Hydrogen, none of which are likely to be detected). However, a side-effect of being covertly infiltrated by these alien general-purpose nanites is that the nanites will also implement unrevoked prior programming that instructs them to look after the health of whatever organism into which they are inserted. The nanites will avoid an uninfiltrated animal that humans are observing, but if they don't believe that any human is paying much attention - and the nanites are individually pretty smart and they all talk to one another, so there is a good chance that they'll know - they'll infiltrate that animal too. After all, if a canary escapes, humans would expect it to stay in the vicinity for a while.
Observation means that either a human or a human-made device physically restrains the animal, a human or a human device spends a significant amount of time watching the animal, such that another human would consider the first person or device to be watching that animal, (really watching it, as in "it is part of my research project" watching, not just casually observing it since it lives nearby) or a human or human-made device attaches an active tracking device.
Passive tracking devices such as leg bands do not count as ongoing observation. Any nanites present would self-destruct on capture of an animal, but after being banded or otherwise marked, if it appears that no great amount of attention is being directed toward the animal, then it will be re-infiltrated.
The nanites that are infiltrating the world's birds and bats are AI spies. They are individually as intelligent as a human, but have no sense of self-preservation. While their orders dictate that they must prevent discovery by humans, they must also observe humans while their host animals continue to act normally as far as possible. Since the act of observing humans leaves their hosts able to be observed by humans in turn, they cannot simply self-destruct any time a human looks at them sideways, since that would leave them unable to fulfil their mission. They are smart enough to know when a human is paying attention to their host with a view to capturing it, either now or potentially in the future, or is looking for any unusual behaviour or traits. Active tracking devices that rely upon GPS or RF broadcast and enable location of the animal at a distance are considered to be ongoing observation.
Since pretty much every non-domesticated bird or bat has been infiltrated, the loss of a few here or there if humans catch and active-tag some is acceptable, but if the opportunity for unobserved reinfiltration occurs, it will be taken.
If humans begin to capture birds or bats en-masse, the nanites will probably assist a few to escape. These animals aren't all that smart naturally, but they are smart enough to recognise a new "predator" eventually, so their fleeing wouldn't be considered unusual.
TL,DR: the nanites are each as smart as a human and they all talk to each other, and if they think that humans are too interested in an uninfiltrated potential host, they won't go there, and they'll self-destruct if humans appear to be approaching a position where there is any possibility that they could be discovered within that host by humans.
However, would we even notice? If so, how long might it take? Could we notice in under 5 years?
Extra credit: Can anyone deduce why Hafnium, Tantalum, Silver, Holmium and Lutetium in addition to elements C, N, O & H and other light elements?