(This is all based on non-CA system.)
I had a useless fob I bought off ebay for the roundel emblem. I decided to take it apart. I unsoldered the battery from the board and stuck the board into the slot. Nothing. If you stick the valet key itself (removed from its fob-shaped cover) it will activate the car, but the bare board did not. This would seem to support the "battery needed" hypothesis, but I took it one step further. I put the board back in the FOB case as best I could (it got pretty destroyed getting it apart) without the battery and it DID
acknowledge its presence with the circle/slash key icon on the dash.
Conclusion: Battery is not needed, but with the RFID in a normal FOB, the FOB shell must be present to push a switch or something mechanical to let the system know that it is there. The valet key doesn't seem to have this restriction and will activate the system by just being present in the FOB slot.