A/N: This is to be read as two phone conversations, which is a bit different from my usual format, So I thought I'd give you a heads-up.
"Hullo?"
"Astrid, we need to talk."
"Yes, Lestrade?"
"May I ask why there are a pair of your underwear at my house?"
"Excuse me?"
"Well, they aren't mine, and I haven't had a woman over to the flat in an embarrassingly long time, as I'm sure you can deduce. The only female to come near my flat in months has been you, so they must be yours."
"Lestrade, why would I have left…"
"Astrid?"
"…Describe them."
"What?"
"Describe the newly discovered undergarments in detail."
"Um…they're black. And lacy. And they've got these sort of frilly bits on them. And a bow. And they're black."
"…Why don't you call Sherlock and have him explain how my underwear ended up at your flat."
"Yes, Lestrade."
"Um, Sherlock, I've sort of an odd question."
"I'm sure. Pray ask it."
"Did you put…Astrid's underwear in my flat?"
"Yes."
"Why?"
"They were entirely inappropriate for a girl her age."
"You're kidding."
"What?"
"You didn't approve of her undergarments so you break into my house and leave them there."
"Yes. I fail to see why you're struggling with this concept."
"Because, Sherlock, it never occurred to you how it looks for me to have a teenage girl's pants in my bedroom!"
"How is that?"
"Like I'm bloody sleeping with her or something, Sherlock, good Lord!"
"Seeing as it's been quite a while since you had anyone in your bedroom besides yourself, I estimated the danger to be minimal for everyone involved."
"…Sherlock. I really, really shouldn't have to establish this, but from now on, my flat is off-limits as a place to hide things of Astrid's that you don't approve of."
"…"
"Sherlock."
"Fine."