Finding a computer with wireless internet??

Ok, so this is purely out of curiosity's sake, anyway, so my room mate had his laptop stolen, and it was recovered because the guy was on the wireless network at school, and they located the guy using it with nothing but his MAC address.... any clue on how they would have done that? *shrugs* Its kinda cool, I mean, I know it can be done with cellphones and crap, but any idea on the technological backings to that? This is incredibly fascinating to me, or do you think they had another way...

<making it up as I go>
Maybe they listed the computers on the network - assume the mac address is listed as part of the info for the computers. From there find out which access point (tower) it is connected to or track the points if it is moving? Since there is only a certain distance from the access point the computer can work on...
</making it up as I go>

Ya, I figured they'd know like, a building, since each building has an access point, so that would be the one they connect to, but still, they knew exactly where he was, and thats almost creepy

My bet is on them knowing the MAC address and then using that to filter the traffic out and see what and where the computer was going. Once you have all that traffic there is plenty of plain text stuff you can see. You see that computer check an email account for Joe Smith and you know Joe Smith has the computer.