After an absence of around a week (of which I hope you’ll forgive me) I intend to come back with something a bit special.
German mines have locker rooms that are adorned with what look to be birdcages. These are actually hooks for the miners to hang their clothes on while they are at work. The cages are then hoisted up to the ceiling. This indicates that the miner is at work in the mine. Obviously when they return the birdcages are dropped and clothing retrieved. This was a simple yet very elegant way to discover if someone on the shift was left down the mine.