An alarm system is disclosed for warning personnel at a loading dock that a trailer being loaded has crept away from the dock. In a preferred embodiment, the alarm comprises a box-like housing that can be bolted onto the dock; a clamp that is magnetically held against the outside of the housing and which can be removed for quick attachment to a nearby trailer; and, a spring-loaded cord that is spool fed from inside the housing and connected to the clamp. After the clamp has been attached to the trailer, the alarm automatically arms itself through the use of a novel timing mechanism and spring-loaded actuating arm inside the housing. If the trailer then creeps, it tugs the cord, which causes the arm to trigger the alarm. The alarm stays on but can be automatically shut off by simply replacing the clamp back onto the housing.
