This has been used for a long, long time in mines, where communication can be very difficult. If there's a danger, e.g. gas buildup, trace amounts of mercapta

This has been used for a long, long time in mines, where communication can be very difficult. If there's a danger, e.g. gas buildup, trace amounts of mercaptan are added to the ventilation system. Miners smelling that awful rotten scent know to retreat to the nearest shelter area in the mine. To signal an all-clear, they replace mercaptan with wintergreen.