Line Finder (Proximity Sensor) will drain a 9 volt battery overnight even when not in use. It is assumed to be completely powered off, only drawing power from the battery during testing. Possible leakage current/short circuit or phantom load even when not in use. Now I have to unplug the battery when I'm done and I always have a spare battery (and they're not cheap). Otherwise it is cheap, functional, but the sensitivity could be better.