electric test problems, safety?

All of the wiring for the direct-wired undercabinet lights on one side of our newly remodeled kitchen was staying hot, regardless of the action of the light switch. Even the follower in the string of lights, supposedly just a loop of wire hanging in the wall, were showing hot. The only way to turn them off was to trip the GFCI to interrupt the >outlets< on that side of the kitchen. I was utterly mystified---I couldn't see any way to make the circut do that, short of actually wiring it for that behavior on purpose. Just before I called the electrician to come back and figure out what he had done wrong, I tried one more thing:
I was testing for current with a non-contact, inductive tester. The electrician had stapled one of the followers in the string of lights alongside the cable for the nearby outlet, literally with the same set of staples. Further up the line, the cables for the lights were twisted together for convenience. It looks like the light wiring was acting as an antenna, carrying just enough induced energy to activate the inductive tester. Testing with a directly connected test light , I found that the light wiring worked exactly as expected, turning just one hot lead on and off with the switch.
Apparently the results from these inductive circuit testers should be taken as a quick "suggestion" of what's going on. Does anyone have any other observations about the safety and effectiveness of thsse non-contact testers?
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I quit using them when one gave me an false positive on an fuse. During the same shift we were working on some 400 hz stuff and my buddy was on an ladder inspecting the wires in an junction box the tester gave me a false positive on his knee for 400 hz. Mine has been in the tool box ever since.
Check the switch make up and you will find the problem
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Funny you should mention 400hz, similar issues can be had with high impedance multi-meters, can indicate an induced voltage on the line. This happened to me when I was checking some 400 hz lines in an aircraft. My brother who worked as an Avionics tech at a major airline company said they banned various models of high impedance meters due to this issues. Never thought about the inductive testers much but never really used any but once and that one was borrowed. Guess I'm staying with the good old an true analog meter I have been using for years.
MC

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