A Bit of Scarlet (слэш)

"When I first met John Watson, I had just moved to Baker Street and he had been home from Afghanistan nearly six months. We'd both been evicted about the same time: I for a disagreement with my landlord about chemical experiments and he for a plain and simple lack of cash. My brother had picked up the pieces for me; his had not."
In which Watson turns to prostitution after his return from Afghanistan, like plenty of other British soldiers, but of course Holmes finds him anyway.
"A bit of scarlet" is late 19th/early 20th century homosexual slang for sex with a soldier. OBVIOUSLY.