It would be stored in ARP tables on devices in the same network as it (any device, computers, routers, etc.), or in the MAC address table of the switch it's directly connected to.
ARP caches on PCs tend to expire very quickly (less than a minute in Windows Vista/7), so you're not going to find anything there; the same is true for MAC address tables on switches (default timeout is 5 minutes for Cisco), and you would anyway need a managed switch (and access to it) in order to inspect its MAC table.
The only place you'll likely find a MAC address after 2 hours is in the logs or configuration for your DHCP server -- and that's assuming that the host is configured using DHCP. Otherwise, I'm sorry, but I don't think you will be able to find anything after 2 hours.