When I first got my Ambient WH31L I was getting 1 or 2 false positives per day with the default switch settings. I changed the switches to the medium sensitivity and it was good for over a year. But all of a sudden it again started showing 1 or 2 false positives each day. I have it installed outside about 30 or 40 feet from a neighbor's large utility shed. I suspected the furnace, a compressor or some other electric device in the shed was causing the false positives so I moved the WH31L to another spot in my back yard about 100 feet from its original location. Sure enough, the false positives have mostly stopped (still get an occasional one every few days).
Just as a reminder to those changing switch settings on a WH57 or Wh31L, you need to reset the device by removing the batteries, then reinstalling them after changing the switches. If you just change switch settings with the batteries installed, the change doesn't take. It is something I learned the hard way.