Many of them make a little noise right before the picture is taken, I have seen them react to it. Scent is another thing.
I remember one of the first one's I bought many years ago was the old 35mm with a flash. I was so excited because I was going to get that picture of the elusive bucks that I just knew showed up once I left. Well, I got it all set up and was in the stand. It started to get late and I had 8-10 deer around when a bright white light lit up the world!!! Scared the begeezus out me and the deer (I had forgot it was there). I might as well had shot at them. Ended up getting plenty of pictures on it, but man that first time was a circus
Never did find that elusive night time buck either