This might be really obvious but did you have anything slightly weighty sitting on the passenger seat? I remember pulling over once because of a random non-stop beeping only to discover my bag was triggering the seatbelt alarm. I felt like a right doofus.
i cant load the video but in my st-x i had a strange beep noise happening radomly, after months of unplugging different things to see if it would change it ended up being the speed warnings in the headunit, turned them off and no more beeps.