well...I'll put it this way...
It would only take a second for someone to slit a hole in the side of your bag to rummage thru it. You wouldn't even notice.
If you keep your money and whatever in a money belt there is no way someone is going to get into it without you knowing!
I'd invest in a money belt. They are cheap and will probably save you some worrying about your cash, passport and tickets stashed in your bag.