So I have the same issue! Although every time it has taken off I've been able to find it. It's a tiny beveled rectangular piece with rubber straps on the back that are extended a bit outwards so you can fold one side inwards after placing another into the area where the button lays at a 45 degree angle(or less). Once it has been folded you can then push down as if you are holding it for a power off until the phone either turns on or prompts your power menu options. Let go and see if this worked by performing several of your daily tasks then locking your screen. If this keeps happening you may wish to consider adding a TINY amount of superglue(or something) where the rubber meets the plastic button before placing it in as described above(obviously let it dry first!), although I have yet to try this myself and I may next time it happens!