I eventually managed to track down the original owner and had them unlock the devices. If I hadn't, these phones would be ewaste.

What bothers me is the solution is simple, when a manual factory reset is done, have the phone ping google and start a 1 week countdown. Google can then email the original owner and ask if they have had their phone stolen. If they reply yes then the phone is locked. If they reply no or have no response then the phone unlocks.