Mysterious Beeping - Seatbelt?

So, driving home in the rain tonight my Prius scared the hell out of me with some loud, continuous beeping after driving for a few minutes, which then increased in volume after a minute or two. In total it probably lasted about four minutes. I couldn't pull over where I was, but when I stopped at a red light, the beeping stopped too and never re-started the rest of the way home.

There was no warning light, it's been recently serviced and had the hybrid system checked. The only thing that changed was the display switched to showing that there was weight in the back seat (my dog). My dog had her seatbelt on, but maybe it was the wrong one for the weight sensor?

Does this sound like a seatbelt thing, with the increased urgency and persistence of beeps? Every other time I've had seatbelt warning it hasn't been that persistent or long-lasting. Seriously, the beeping was far more likely to cause an accident than anything else at this point!

 

Shouldn't that have a warning light of some kind? Or does it not?