Family sharing - child has app in screentime usage stats

I was checking my very untrustworthy 13 yo son's iphone today which has some strict controls. He has ADHD so I have to limit him as he cannot limit himself currently. I found something under his screentime stats today called "ScreenTimeUnlock". I also found a note written last week with my screentime passcode on it. Up until then, he couldn't bypass the restrictions. Today he played a game for 2 hours in secret.

Does anyone know what this app is (it has the same icon as GameOverlayUI). There is no app visible as a tile anywhere on the phone.

I've now disallowed installing of apps in restrictions but this makes updating apps hard when he's not at home for long periods. If he has some kind of unlocker, won't he just crack my new code immediately?

Surely someone else has come across this? I can't find anything about it on the web.

Any help appreciated. Managing a child that has no off switch or self discipline and requires constant stimulation is a challenge. Trying to keep one step ahead of him is increasingly challenging.