Second attempt posting this prompt, I don't think the first time worked.

John's having a friendly chat with a patient who keeps going on about a certain consulting detective she met earlier on one of his cases. Obviously she doesn't realise she's talking to Sherlock's flatmate/colleague/best friend, since John wasn't there, but he just listens and plays along. But then she tells John about how flirted with Sherlock (she's a very chatty person) but he turned down her advances because he said he was already in a committed relationship.

Cue John's confusion (after the patient leaves, of course) about who exactly Sherlock is in this "relationship" in - possibly realising it's him - and confronting Sherlock about it.