Apparatus for detecting and signaling the presence of movement and the absence of movement of an operator moving randomly in space and time thereby to detect when the operator is immobile comprising, a portable transmitter carried in use by an operator moving randomly in space and time, said transmitter having means for transmitting a carrier frequency continuously while in use, a detector associated with said transmitter for detecting the random movement of said operator comprising sensing means having means defining a radiation field, an element freely movable randomly in three dimensions in said field in response to random movement of said operator and constrained to move in said field, said element being configured to be physically unstable to preclude substantially a physically stable condition of the element, switch means detecting and responsive to a varying extent of masking of said field by said element upon movement thereof to interrupt emission of said carrier frequency signal when said operator is substantially immobile and for re-establishing said emission of said carrier frequency signal upon renewed movement of said operator, a portable receiver unit positioned in an area for receiving said carrier frequency signal, and means on said receiver unit for activating an alarm after a period of continuous absence of emission of said carrier frequency signal is detected by said receiver unit.