this was one of the reasons I chose the Lowepro Flipside - it opens only on the side which goes against my back. So as long as I'm wearing the backpack, there is no risk that someone will be able to open it.
Except with a razor knife.
Correct ... but the flipsides do act as a safety feature in a rudimentary sort of a way though.
However, if things have gone so far that I'm confronted with someone with a razor knife, I would probably be wise to meekly hand over my gear - a bag, howsoever safe, won't make a difference.
5D3, 6D, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 600RT x 4
I come here to learn something new, not to learn how bad my gear is - I know that already ;-)!