Well, it is not exactly camouflage, but the impact with terrestrial air; it is like the two waves thah a ship produces...the foam waves at every side and also there you could see in distance something like a plane
For the UFOs we know they use "concealment devices" and with those, you can't see them 'cause perception of sight is different. Do you remember Star Trek movies and the hidden Klingon ship?
So, if someone would say to you "oh you are dreming" well, remember them that there are evidences that other races came to visit this planent 15,000 years ago before the man and from that time they come back to see how we proceed, in awaiting to reveal themselves to the terrestrial population.
[Currently this revealing, is only for certain subjects chosen by them]