If there is a bulge before the vent and you hold them, the warmth and movement combined will cause them to use the washroom on you.
My snakes pee... A lot! It's annoying but I guess it's the joy of owning such animals.I know when I had a bearded dragon, as soon as I would drop him in warm water it was like instant poop.. Its weird! I see you have a few bp's and are aware but, I'd just make sure the temps in the cages are where they're supposed to be. Maybe you are warmer than the cage and it's causing it.
Not a professional by any means, just my opnion and reading other threads like this.
There are a few threads around here involving the same issue you're having.
Goodluck!