I'd leave it alone.. if I knew who it belonged to I'd email or message them to let them know.. if they asked me to lock it for them I would.
Much depends where you work... if you happen to work in a home for delinquents, then chances are it won't be safe... if you happen to work in a nunnery, then I guess it would be safe..but that's a huge generalisation of course :P
I work at McDonald's. It's just in the women's locker room. Not many lockers there, but people go there to lock up their stuff, change, go to the bathroom, hang up their stuff, etc. Not everyone even has a lock or uses one.
AG, I am going to guess that you are okay. I myself am VERY careful about locking everything, but every time I forget it seems like nothing ever happens, it's all intact when I come back!