The trumpeting coming from the loo sounds distinctly like someone blowing their nose, but Sherlock's in the shower. There's no way a tissue would stand up to that sort of environment... Concerned, John sticks his head into the room.
"You alright, Sherlock? What was that noise?"
Sherlock peers out from behind the curtain.
"Hm? Oh, nothing, I was just blowing my nose."
"With what? Did you bring a handkerchief in there?" John sounds incredulous, wary.
"No, of course not. I just used my hand and washed it down the drain."
"Sherlock! That's revolting!"
"Why? It's just phlegm."
"It's gross."
"This from a man who was in the military, and proceeded on to a career that regularly involves people vomiting or voiding themselves. I had no idea you were so squeamish."
"It'd be fine if it were just you, but I use that shower too!"
"John, I had no idea you were so squeamish about bodily fluids..." Sherlock's voice is teasing now. "Shall I invest in some condoms?"
John groans, rubbing his brow in frustration.
"It's unsanitary, that's what it is."
"I suppose now would be a bad time to mention I occasionally urinate down the drain while I'm alone in here?"
John's face blanches and he leans against the doorframe, realising this is a lost battle. "Just clean the bloody bathtub!"