I read cytomegalovirus can be transported in urine. what if it's such a small amount of urine that it's invisible and just a one time exposure? can that be enough to absorb in the mucosal membrane of the mouth?

Cytomegalovirus: At your age (36) you likely already have CMV, usually acquired in childhood. If so, you are immune to new infection. CMV is mostly transmitted by saliva, blood, or sexual fluids. Urine can be a source, but a miniscule urine exposure like this is very low risk, probably zero. In otherwise healthy people, most new CMV infections cause no symptoms and are harmless anyway. Nothing to worry about!