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Seen a couple of similar issues but never a meaningful solution posted...
AUDIO CLIP ATTACHED
I have a 2007 civic and it is randomly, but OFTEN, producing this double beep when in motion - stops when car stops moving. It comes from the same beep speaker as the seatbelt sensor (which is a different type of beep and I know this because when I take my seatbelt off while driving the seatbelt beep takes over that speaker from the mystery beep for like 10 seconds then goes back to the mystery beep).

There are no lights on the board and my OBD2 does not detect any faults. I strongly do not believe it to be a passenger seat or door or trunk sensor (those indicator lights are working on the board when open) but also I can't prove it’s not.

I thought I had traced it to an A/C refrigerant leak because when I topped off with more refrigerant it stopped for the whole winter. But it’s back now and more refrigerant is not solving this time. I am FLUMMOXED. Can anyone help?!
In the audio clip (and always) the beep starts around 3 MPH, stops when I drop below that, and starts again at/above 3 MPH. The beep might start randomly in the middle of me at speed, it might stop at any random moment and not do it for the rest of the day, it might do it ALL DAY LONG - no rhyme or reason. A mechanic had it for a day and could also not determine the provenance.
I have no aftermarket anything installed.

I also promise to post the solution, should I encounter one.