The word safe is a relative term in espionage. Handlers who often work from safe houses know much can go wrong because of both enemies and local authorities. When a safe house is compromised, others who may be arriving must be warned in a way only they will understand. A simple signal is best—all the windows thrown open, a shutter removed, or a brightly colored towel hung in a window. Seeing a predetermined sign, anyone approaching would know to immediately clear the area and call their embassy for further instructions.