I used to use this crazy little piece of wire to search for hidden pipes, duct work, etc, above ceilings when ever I had to install recess lights in a finished ceiling. I would take a piece of #10 copper about 8-12" long and form in into a Long "Z" Like this:
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The length of the first horizontal piece is determined by the size of the recess can. You make the copper wire 3" for a 6" can or 2" for a 4" can.
Then you lay out your pattern on the ceiling. poke a small hole where you want your first can to go, stick the wire inside the hole and spin it like a helicopter blade. Then as you spin, you raise and lower the wire up and down to "see/feel" if there are any obstructions in the way. If you hit a duct or pipe you will feel and hear a metal TINK. If you are to close to a rafter or floor joist the wire will stop spinning and you will be able to tell where the beam is.
I hope this picture comes through.
