Lock cylinder with key recognition

You could also consider another way: that the key must be at a precisely defined place within a certain amount of time. At my workplace, there is a box where the keys of the rental and company cars are kept. The box is opened with your stamp chip. Then you select the number of the key. The key fob looks like a 6.3mm jack plug, which allows the system to recognize where a key is available. Something like that could be built. Door contact on the front door, when opened a timer starts running, provided the key is not in the key box. After a certain time, an alarm goes off. If you open the front door when guests arrive, nothing happens because the key is in the key box. Just an idea, no idea if it is practically feasible.

I think this is much more complicated than simply taking the key out of the lock, and that is already a problem, as it seems.

All easy - after the first goodnight visit with a gun held at it, she'll get out of the habit...

Or maybe a keyless lock with a code or fingerprint?

Just an idea, no idea if it is practically feasible.

Yes, it is. I think the idea is very good, to rethink the warning from "key is still outside" to "key is not inside." My preferred designated place for the key would be the lock (inside). It would need to be monitored whether the key has reached there within x seconds after opening. The door would need to be shielded on the outside so that the receiver does not "detect" the RFID tag as being in the desired area when it is also inserted outside. In other words: one of the two door sides would need a shielding plate so that the OK signal only occurs inside.

Hello,
Really let go of the key.

In addition to the normal key, we have received handles for our front door, like a car handle, for example.

Our children use it now.

No key can get stuck there.

Best regards
Birgit