Try:
- /seen
- /whois
- /lb tb (and smack stuff)
- /region info
- look around for signs of recent use
"Player not found" usually means they haven't ever logged into the server, though sometimes it say that for a couple other odd reasons. If all of the owners of a region (protected town) have been offline for over 2 months, the existing members are first in line for ownership, and beyond that yes you can claim it. If the owner of an unprotected town has been offline for over 2 months, you can salvage it or claim it.