Is it possible to detect the heartbeat of a person within a proximity of few centimeters away from the body? The scope is to detect the removal of a device which must be kept near the body all the time, but in most of the cases, the person in unaware of the presence of the device.
What if the harness had electrical contact through the fastener? The fastener would open contact when the harness was removed. Another thing to measure is magnetic field direction. Children are always moving around. If the vector of the earth's magnetic field sudden stayed stationary you would know the device had fallen off the child.
......... Children are always moving around. If the vector of the earth's magnetic field sudden stayed stationary you would know the device had fallen off the child.