undefined> What if it turns itself on while stored in a backpack?

you are asking the right question

the answer is, do not store in your backpack until you use a strategy to eliminate accidental discharge

on some lights, if the threads are anodised, not shiny bare metal, we can break the circuit by unscrewing the head or tail. So the button will not turn the light on.

to find out if that works with your light, turn it on, and then unscrew either end about 1/4 turn.. If the light turns off and the button does not turn it on, you can now store your light in your backpack safely..