Elementary crossover, whoops! Wrote this on Tumblr a while back and realised with a few tweaks, it would be a workable drabble :)
They'd been in New York for a few days now on a case. John was exhausted and irritable. He'd left Sherlock in their room and headed down to one of the ubiquitous Starbucks that seemed to be on every corner. He'd just sat down with his tea (watery, bitter, but better than nothing) when a lovely woman sat down next to him. He smiled, nodding.
"John Watson. Can I help you?"
The woman's eyes bulged briefly, shocked. "J—Joan. Watson. And, I'm not sure."
They both let out nervous giggles, the coincidence too amusing to ignore.
"I've noticed you hovering around at crime scenes. I…" she paused, unwilling to reveal too much. "I work with the cops. Well... someone I know does. But you don't seem to be affiliated with the police..."
John sighed, rubbing his forehead. "It's complicated. We're - my partner - is a consulting detective."
Joan looked startled again. "Are you pulling my leg?"
"I wish I were."
Suddenly, John's phone buzzed.
Bored! Come back to the hotel. -SH
"I'm sorry, Joan. I need to get back."
She smiled and glanced towards the back door, where a scruffy and irritable-looking man stood looming and gesturing at her.
"Not a problem, I think I'm also being summoned. Nice meeting you, Watson."
"You too, Watson. See you soon, I bet."