Using Android Device Manager to determine state of stolen device My phone was stolen over a month ago, during which time I had already sent a lock request to the phone via ADM. 

The device still appears on my list of 'recently used devices' although it is clear it has not been used since being stolen. I am still able to send lock requests though.

I am curious: If the thief had somehow hard-reset the phone back to factory settings, wouldn't it automatically be removed from my list of devices? Or will it simply remain listed here indefinitely until I choose to remove it myself?
The List will remain same even if the thief has done a hard factory Restore So you cannot trace the phone again if he had done a factory reset or flashed a custom/stock OS again Android Device manager is helpful only if he hasn't done a reset to your device better book a complaint using your IMEI number to Police they can help you :)