The thing with RI is that if you were at such an advance stage that mucus would just fall out of your BP's mouth, you would be at a VERY advance stage which means you would have noticed earlier signs such as crackling sound/whisling , open mouth breathing, rubbing etc.

Mucus just does not appear overnight, another thing the consistency between mucus and drool (from an animal drinking and being startled) is very different. Finally you mentioned a clean mouth.

I would keep an eye on the animal over the next few days and if you really have doubt take the animal to a vet and have a culture done.