There might be an RFID tag inside the handset equipment (phone), for the purpose of inventory tracking at the manufacturer. Though to get higher distances (like over a meter) one has to get quite directional with the scanning.

Chemist, can you describe the equipment used for detecting the powered down phones? Also, was it able to identify the phone, or just know there was a phone somewhere? The latter could even be an antenna resonance trick of some kind, but I doubt it would be possible to identify/discern two different phones of same model with that.

Edit:
On the original topic, for the ~20 years cellphones have been available to average consumers, there has been rumors that the government/teleoperator/CIA/Mafia has ability to activate a cellphone remotely for tracking or eavesdropping purposes even when seemingly powered off, the GPS location seems like a natural continuity for that. My honest guess is that it is not possible on the stock equipment out from the manufacturer. However, as with all IT security, if the hostile party can get physical access to your device, at any point of time, you're screwed. One way or other.

I have no reason to be too worried about this stuff. If someone truly has, my advice is: Simply, and completely, stay away from (a) technology, (b) other people.