this happened to me. I legit freaked out. I was on a bus and snoozed for a literal 30 seconds and the guy (or girl, idk) was GONE >.< Turns out the security camera on the bus was broken, too >.<
My solution: put the backpack or bag in front of you and wrap one of the straps around your arm. This way you are literally holding it and they can't get into it behind ur back or yank it from you.