Fortunately for Lily, the door of the apartment is already open because of Andromeda.

"Not what it looks like? Really? Because it looks to me like you two are still behaving exactly the way you did in Hogwarts despite your apparent age and experience!" Not that this is unexpected from Sirius, given where a good chunk of his adulthood took place ... but still. Explanations shouldn't always have to be excuses.

"Sirius - I'm not stepping over this floor until it's anything less than the death trap it already is. Both of you - put your wands down, or I swear to Merlin I am going to come over there and punch you both in the noses." Already-bleeding or not, Severus. There is muttering, mostly about old grudges and bloody stupid immature ... well, you get the point. She does start to slowly try and make her way over to where Severus is hunched, however.