Maybe Lestrade suspects Sherlock is hiding drugs again and alerts Mycroft, maybe there's some other reason, but, anyway, Greg and Mycroft end up going to 221B while Sherlock and John are away. They don't find what they seek in case of drugs suspicions. Instead, they get trapped in the flat hiding, when John and Sherlock suddenly return.
Sherlock and John start making love (top!John, please). Neither (not even Sherlock) suspect they have unwilling spectators (maybe Greg and Mycroft hide in the bathroom or behind the sofa in the living room (while sex can take place on that sofa).
This is beyond embarrassing and awkward for both Greg and Mycroft. Whether they get discovered (which makes everything absolutely mortifying for all four of them, and John and Sherlock can be pretty angry if you prefer) or manage to flee from the flat once they have a chance to do so unnoticed (Sherlock can deduce it later anyway, I don't really care), is up to you.
Maybe the experience was intimate enough for Greg and Mycroft to end up together somehow.
Bonus points for slightly kinky sex. For example, John convinces Sherlock to sit down on his face and opens him up with his tongue, before he starts fucking him. Maybe they are a new couple and Sherlock is a bit shy about things like that.
Mycroft says: "Kindness of strangers". I've got a feeling it's sarcasm...
John/Sherlock. Possible pre-Greg/Mycroft. Accidental voyeurism.
Sherlock and John start making love (top!John, please). Neither (not even Sherlock) suspect they have unwilling spectators (maybe Greg and Mycroft hide in the bathroom or behind the sofa in the living room (while sex can take place on that sofa).
This is beyond embarrassing and awkward for both Greg and Mycroft. Whether they get discovered (which makes everything absolutely mortifying for all four of them, and John and Sherlock can be pretty angry if you prefer) or manage to flee from the flat once they have a chance to do so unnoticed (Sherlock can deduce it later anyway, I don't really care), is up to you.
Maybe the experience was intimate enough for Greg and Mycroft to end up together somehow.
Bonus points for slightly kinky sex. For example, John convinces Sherlock to sit down on his face and opens him up with his tongue, before he starts fucking him. Maybe they are a new couple and Sherlock is a bit shy about things like that.
Mycroft says: "Kindness of strangers". I've got a feeling it's sarcasm...