Rewatching episode one yet again I noticed it.
Lestrade's magical powers over Sherlock's front door.
Sherlock and John meet at 221B in order for John to inspect the flat. They knock on the door, Mrs Hudson answers and lets them in. The door closes firmly behind them all and we see a close-up of the number and the knocker.
Upstairs they chat and Mrs H, reading the paper, mentions the mystery of the 'serial suicides'. Sherlock sees the flashing light of a police car drawn up outside and deduces that another one has happened. As he speaks, Lovely Lestrade appears bounding up the stairs and into the flat.
How did he get in?
We saw earlier that the door was firmly shut.
Lestrade doesn't have a key. Even Sherlock doesn't have a key yet. Mrs Hudson didn't let him in. There's nobody living in 221C to let him in. The skull didn't let him in.
It's a bit of a surprise that he even knows where to go, seeing that Sherlock has only just moved in on the expectation that he can take the flat because John *will* want to live there too, but I suppose we can presume that just as Sherlock is sure that John will accept, we can also presume that he has texted his new address to Lestrade.
But that still doesn't solve the mystery of how Lestrade got into the flat.
His visit doesn't warrant barging down the door and that hasn't happened because it's all in one piece not long afterwards when Sherlock and John leave together, nor does Mrs H start fussing about any mess caused when she goes downstairs herself.
No doubt Lestrade could have picked the lock, but again the situation doesn't require it.
I think the only answer to the conundrum is that DI Lestrade has magical powers which enable him to open front doors.
Now I'm sitting here waiting for him to open mine.
Maybe I'll write some Lestrade/Door in the meantime.
ETA: Which I did. And it is
here.