In season 1 Episode 4 of Fringe Dunham makes a revelation about the Observers with Peter and shows a picture of an Observer in a hospital:
She then shows the same picture to Boyles in his office and the Observer is no longer there:
Can the observers not be seen by everyone? Why was he missing in the second photo?
I don't think those screenshots are in the same moment in time, everyone is in different positions. He may have just moved when the picture was taken.
I don't remember any reference in the show that says they cannot be seen by some people.
They have ordered meals in coffee shops and have been photographed quite a few times; true invisibility seems right out. If they possess an ability to make people unaware of them, it hasn't been directly mentioned or even hypothesized. That Fringe division isn't more worried about them is odd -- I would think they'd be considered very threatening.
