I think that must be a BT only nickname as engineers I've dealt with from other Telcos either called it a TDR or a 'tester', not very imaginative! I wonder how the name came about? Maybe because the process is like sending something the length of the cable to check it and given cables are often subterranian a mole is the most likely creature. Although ferrets tunnel as well, if I ever get a TDR machine I'm going to call it 'the Ferret'.

EDIT: It appears the device the other guys were using was in fact an OTDR. But I suppose it does the same sort of job, just with a different medium.