I always thought in that scene where Dutch is hugging a tree and the Predator is right behind him that he should be able to smell him.
All that dripping and slime on them, mouth full of spit etc, that's bound to smell foul, right?
I bet Predators have a distinctive smell about them. Do the comics ever mention that?
Likewise, do Predators have a sense of smell? Jungle Hunter should have been able to smell Dutch. Days without a shower in the jungle? You don't need infrared to spot him.