2004 Ford F-150 Back up sensor problems

I have a 2004 Ford F-150. Recently, I had a back-up sensor system installed. It worked fine for a couple weeks, and then decided sound every time I would back up even though there was absolutely nothing behind me. I took the truck back to the service center that initially installed it, and they reprogrammed something. Same thing - after a couple weeks of working fine. The have since replaced that unit three times - then replaced it with a different unit - a DT 506 - no brand name. Same situation. The last two times it started doing this was after I had lowered the tail gate. Putting the tailgate up makes no difference. The sound is constant when the truck is in reverse.

Any idea what is going on with this? I have several friends with the same backup units as I initially had and were installed by the same company, and none of them have ever experienced this.