They'd been back from the case in Dartmoor for a few days when it happened. John was minding his own business, checking his email, when Sherlock cleared his throat.
"John?"
John closed the lid of the laptop, studying Sherlock's face. There was a flush across his cheeks, a clear sign that whatever he was thinking about, it was exciting him.
"What's up, Sherlock?"
"When you pulled rank. At Baskerville, I mean. Is that... I mean... can you do that at will?"
John raised an eyebrow. "Are you asking if I can fall back into army mode for a case or something?"
At this, Sherlock flushed even deeper. "Not a case, precisely."
"Sherlock..." John's voice was teasing, slightly rough around the edges. "Are you telling me you've got some kind of military kink?"
"I wouldn't call it a military kink. Not entirely. More of a Captain Watson kink, specifically."
John got up, crossed the sitting room and ruffled Sherlock's hair.
"I think I still have a few things packed up in boxes, let me go check." He smirked to himself, knowing full well he still had a full kit of both combat and dress uniforms.
And that was how John ended up sweaty and exhausted, draped across Sherlock's bed in nothing but his dog tags and pair of well-worn old combat boots.