"I'm sure he didn't know he was doing it"

I reckon that's part of the trick to doing it properly. Subconscious connection twixt the nner world and the outer one etc. I don't think the dowsing rod reacts of it's own accord, rather it reacts to tiny trembles in the hands, some of which are being induced by a bit of the brain that isn't conscious, but can detect anomalies in the environment.

Mebbe.