Yea, your guess is as good as mine. I get it from time to time, too.
I've not found a low fluid level, now, once I had the oil level alarm going "HI", but it hasn't done that since I started using 8 (not 8 1/2) quarts at oil changes.
I get the beep at times, too. And I've had two Roadsters, the red one did it once or twice, the Graphite is doing it at least once a MONTH.
Who's guessing? Its in the repair manual, section 8B-2.
I call it a chime, the repair Manual calls it a chime, it is actually a beep and different from the annoying seat belt chime. It probably is just one tone from the seat belt chime.
Leave your door half open and listen.