Agreed. When I first started Uber I was getting really pissed off thinking they were taking pictures and almost said something to someone but it's the sound of the iphone when shutting off.I thought that was what I was hearing too. However, I think that sound is actually from the passenger putting their iPhones to sleep, not taking a picture. It's a very similar sound and I've noticed it happens as they are putting their phones down. That, and the fact that there is no reason anyone would want a picture of me, lol.
I have had passengers taking selfies before but that is really obvious.