It's a backpack. You hang it on the hook on the stall door where you hang your jacket
I think there is less of a likelihood someone is going to jump you for your backpack when you are at a restaurant for lunch than there is for them breaking into a car parked only a dimly lit street for said backpack.
Except that American public toilets have gaps so large between the door and the frame that a thief can pretty much just grab the backpack without opening the door =)
I live in big City America. This is what I did. Hang my backpack on the stall hook as stated or shit with it on my lap. I carried a laptop, DSLR and lens with me for all my college life. I couldn't afford to replace anything if lost during said times as an international student whose credits cost more than 3 times that of a non-international student.
I think there is less of a likelihood someone is going to jump you for your backpack when you are at a restaurant for lunch than there is for them breaking into a car parked only a dimly lit street for said backpack.