It is (or was) a loaner from Pierce at Tower 5 for their unit that had mechanical problems and went back. I heard they have gotten it back from one of the Bat. Captains.
Also, new R4 is not in service yet either. They are waiting on equipment still and then it has to be sent out to Sensible Products to be fitted.
Confirmed with a paramedic that AFD does now have a GMC Sprinter Ambulance but its only used for non-emergency transport, Not sure of what this means for the future if they are ending a partnership with AMR or something like that. He told me that it is not based at any specific station.