IP conflict and mac

[Xaos <xaos () compulink gr>]

I recently came upon a situation with one of my clients where his Active 
Directory Controller would show an IP conflict error message every few 
seconds...
All the workstations machines had DHCP whose range was nowhere near the 
server's ip and all the other server and routers etc. had different 
static ips...
The server system log showed the MAC address of the machine that was 
trying to claim the IP but I didn't find any way to find it on the lan...
I tried several MAC address scanners and the arp -a command (in case 
anything was registered there) but nothing came up...
It turned out it was an old hub some stupid admin had decided to put 
back online but it was pure chance we found that out...

The question remains... sort of checking all mac addresses by hand is 
there an easy way to locate a machine simply by it's mac and nothing else?

Thanx for your time... :-)