I have a suggestion do you still got that house blue print, find the electrical part.
I have used a small 5Vdc Wall-Wart as a "AC power is ON" detector. The transformer in the wall-wart takes care of the AC power isolation. It's output is a "logic level"; 5V when AC is on, 0V when AC is off. You could even power the "push-the-start-button" circuit from the wall-wart - this circuit is perfect in detecting the electrical wires before boring on it.
It requires a sound pulse generator hooked to the line to track, and a signal receiver to detect it, the professional grade of that equipment is very expensive, so I suggest calling your local electricians and see if one of them has that tracking equipment in their inventory.
Its the same type equipment they use to detect underground pipes and wires for for utility digging purposes.