There are, I belive, animals which have only been observed once or twise and never again - despite modern technology. I think one is a pygmy boar, which was seen once in the 50s, and which has since vanished.....
what you seme to forget, is that in the likes of the jungle there are masses of heat sources - not to mention extremely dense foliage that prevents vision or movement. arial surveys are useless, because of this, as well IIRC. you;d also have to keep the entire area under constant surveillance for many months - stuff like bahviour patterns (sleep periods, hibernation, how and where it would hunt, how often it needs to hunt, etc) could all have an effect.
Also, computers have a massive amount of trouble discerning what a viewed object is (there is pretty much no way a computer can currently identify, for example, what the objects are in an image). so you;d have to do it by eye.
Imagine surveying several hundred miles of populated (by animals), dense forest - and trying to detect a couple of unknmow, vague heat sources.
Even if we have sufficiently disscerning technology to identify an unknown lifeform, we still have to identify a way to actually use that technology over a large area. And there's always the maxim that 'abscence of proof is not proof of absence'.