JUst tossing this put there as I had a similar issue after getting acura to replace my seatbelt retractors... The annoying beep may be accompanied by the seatbelt warning light flashing (may not, but that is the systemn the beep is attached to)
you can remove the kickplate on the passenger side outer wall.. this covers the seatbelt control unit.. unplufg the green connector and see if the beeping stops. If it does you know it is part of the seatbelt unit.
If you have determined that this is where the issue lies, then the next step is to determine if the seatbelt actuator switch which is built into the power door latch assembly has gone faulty.
the switch is located up behind the circular latch assembly that actually secures your door shut. this same switch can cause issue with the seatbelt retractors failing or not working or even getting stuck open or closed. I'll take a picture to post up of what I am referring to later or tomorrow when I have time.
you can use a small precision screwdriver to trigger the switch and see if it shuts the beep up... should also actuate the seatbekts too (unless you have manual belts....