IR Smoke Detectors with IOT would do the trick here and seem a likely choice on account of they didn’t include some bullshit about AI recognition, but it also makes them hilariously easy to game. Just spray some axe near it, you know us teenage boys, we smell. Or hell, dust.
Considering you said you’re german, I think the whole Idea of “Ehrenamt” and subsidiaries of it runs counter to the entire system that has been built. If we monetize everything, I think it’s fine that people get paid for taking time out of their day and bodies to do good shit.
Basically, don’t do unpaid labour in this system?