I would expect that the fantasy 'construction force' would be far quieter and cicumspect, even with the rather wild nature of the of the digging creatures. The use of magic would be almost undetectable if done correctly.
Not really, the vietcong and north vietnamese dug 130miles of tunnels at CuChi during the vietnam war and I dont think they where ever discovered.
Whole communities lived under ground, hundreds of people because it was the only place safe from the B52's.
