For the lack of a term for the phenomenon, Deafumption was coined to describe a phenomenon where one assumes the other to be Deaf. Often stories are funny.
One scenario that can happen in a Deaf community member's life is that when two friends were chatting in ASL. The third guy, who knew the Deaf friend but not the other guy, joined them in conversations. The Deaf friend introduced his friend to the new friend. A while later, the Deaf friend had to leave and said farewell. The two of them stayed and chatted. A while later, somehow one of them realized or discovered that his new friend was hearing and revealed that he was hearing, too. Both hearing men looked at each other and laughed at themselves for assuming each other that they were Deaf.
Another story: sitting at the bar counter, the two men wrote notes back and forth, chatting, obviously because one of them was Deaf. A curious hearing person somehow joined them, writing back and forth with the two of them. As the time came, the Deaf guy had to leave and thanked them. As he left, he turned around to look for the last time. He noticed that two hearing guys were writing back and forth. He chuckled and left. The hearing men wrote back and forth for a while until they realized that either of them was Deaf. They laughed and reversed to vocally speaking.
Another common phenemonon, though, not funny but rather unfortunately audist, is that when a mother and her child talk in ASL. Often, a hearing onlooker would contemplate or sometimes asks a Deaf mother if her child is Deaf. The Deaf mother would reply, no. It is the mother who is Deaf. There are Deaf parents, too. Many are great parents.
Those scenarios are not unusual but also not common. In Deaf culture, introductions often involve information about a new friend -- name, from where, and other relevant information.