This happened to me...not to me, to Max,
yesterday,no foam, just clear fluid, runny nose, once inside, I cleaned his nose and he was fine.If the vet did not find anything wrong, is probably because there isn't. Keep your eyes open and see if this happens again.This season, pollen has been a huge problem every where, also remember, when these babies go for walks they stick their noses everywhere, and their sense of smell is very sensitive.