I hope you find a way to nail it down, my parents had a similar problem at their care home with a resident who often 'wandered' other people's belongings into his own room. In the meantime, I wonder if the staff know that, as it's registered to you (or your dad), it can't be registered to anyone else, and anything that's said directly to it turns up on the app. That may act as a deterrent while you figure out a more physical solution.