In many places, it's OK as long as at least one person knows about it and if the OP takes it with him when he leaves, it wouldn't be a problem. The camera could be in sunglasses, a hat, a small clock, etc.
Our company doesn't allow cameras on site, or mobile phones really. Though most people have camera phones.
You can get sacked if they find out you've been taking pictures on site.
I'm sure they're not going to know I've taken a picture of my locker though.