It would seem that the device could and should remain in an "off" position until needed.
Years ago, spys had a nitro or poison capsule that they could bite down upon in the event they were captured or tortured.
In today's age, a soldier surrounded by enemies could activate the homing device in some manner in case of extreme emergency.
Even if the device only worked for 48 hours, it should be enough to track and locate and rescue.
Lets see how this works in the real world. Some hadji type knows that US soldiers have this device. A head shot is made taking out the US soldier with a clean kill. Body is dragged to an ambush site and the homing device is activated. Rescuers show up and hadji opens up with RPGs. Still think it is a good idea?