Alarm when Sensor is disconnect

Hello!

I would like to program a data field, which just give Alarm if a sensor is disconnected.

I would like to know, if my riding partner is to far behind me: So if his sensor, which I paired, is disconnected, he is to far away.

Is this with a simple data field possible? How can I set a sensor to observe and check if connected or maybe signal strength?

Thank you!