I use an Ideal ABS (Almost Butt Set) 62-180 tester, with a signal probe built in. I was able to find hidden (plastered over)sconce boxes with soldered clusters in them by attaching my signal generator to the old wires and moving the probe end over the wall in the general area where I figured the sconces would be.
Listened for the increase in volume (due to the clusters of wires) and popped a hole. Presto.
Also have used a 61-510 type "circuit identifier" that works reasonable well, but requires some fine tuning occasionally.
Tried to link just to the products, but try this instead:
http://www.idealindustries.com/index.htm The ABS is under Products>Datacomm/LANTEK
The Circuit Identifier is under
http://www.idealindustries.com/tm/ElectricalTesters.nsf/ElectricalTesters?OpenFrameSet
