I'm gonna take a serious note on this. As was brought up earlier, this can be a sign of drug abuse. If that is the case, the missing cash is the least of the problems. Do the electronic thing and find out who. Then find out why. If the person is just being dumb, great. If something more is going on, ya gotta get 'em help fast. Either one of them show any changes in behavior lately?
Good luck and I hope it's just a kid being dumb.