Sherlock narrowed his eyes at Mycroft and Greg, and John bit back a groan.
"Sherlock," he snapped, even as Sherlock tilted his head and stared at the two of them with morbid curiosity.
"They're dating!" Sherlock exclaimed. John winced, even as Mycroft's smirk grew wider.
"Yeah, look, leave them alone, will you? We've finished the case. Let's go home, alright?"
"But, John, John! Look, Mycroft never lets anyone stand in such close proximity to him, even if he's having affairs with them. Are you well, brother?"
John exchanged a long look with Greg.
"Well…" John began. "It is their anniversary."
Sherlock spun to gawk at him. "Their – it's their what? Their anniversary?"
John patted Sherlock's arm. "Can we go home now?"
Sherlock spun back to glare at Mycroft and Greg. "Your anniversary?" he said.
"Three years, actually," Mycroft drawled. John covered his face with his palm.
"Three years," Sherlock said, something approaching hurt in his voice. He huffed. "Well. I wish the both of you the best." His voice wavered on the last word.
John peeked through his fingers. It was Mycroft's turn to appear shocked, although he hid it better.
"Right, er, home. Yes. That's a good idea, come along," Sherlock said, absently glancing at John before striding away.
"You should have told him," John said, not bothering to hide the faint note of accusation. Greg, for his part, did look guilty.
"Hmm. Perhaps I should have," Mycroft mused.
John rolled his eyes and strode after Sherlock, sliding into the taxi he'd hailed. "One of these days I might have to punch that smarmy git you have for a brother," he said.
Sherlock granted him a faint smile. "Did he apologise for hiding it?"
John grinned back. "He admitted that he should have told you earlier." John pulled his phone out of his pocket and played the recording. Sherlock grinned.
"Brilliant blackmail material," Sherlock announced.
"Was it worth pretending to be oblivious for two and a half years?" John asked. "Perhaps I should have actually left you oblivious, instead of telling you."
"Don't be ridiculous John. I would have figured it out sooner than later."
John hid his smirk and nodded sagely. "Of course," he said. "Of course you would."