They're sitting in a café in Vienna when he finally asks that question.

"How did you deduce it was me?"

She pauses halfway through writing Mrs Hudson's postcard, gives him one of her patented 'you always think you're smart' looks.

"Sherlock, your own brother came to my flat. He wouldn't have bothered at all if it didn't have anything to do with you."

"Of course it did," he replies defensively. "You requested my help, I requested Mycroft's."

"He said you had already traced the man yourself. How do you explain that?"

"Marriage annulments are hardly my expertise. He could have it fixed in no time, but he just can't resist a chance to stir up trouble."

"I'm glad he can't. Otherwise, I would have to find a suitable excuse not to marry Tom, and that would be awkward indeed."

He stares at her over the rim of his cup. "You went for the marriage licence. It would seem you were pretty anxious to get married."

"Tom was. I just couldn't find the courage to tell him the truth."

"My brother's visit didn't necessarily mean it was me," he observes, still following his train of thoughts. "It could have been someone I knew, a name I happened to recognize."

"Mycroft also added a curious remark about coincidences. I remember you once told me there's no such thing as coincidences, and I have a feeling that your brother doesn't believe in them either."

"And you got a copy of my birth certificate," he says in disbelief.

"Of course I did. With your history of drug abuse it wasn't such a wild guess, don't you think?"

"I shouldn't have used part of my real name. I've no idea why I did."

A genuine smile tugs at her lips. "Maybe you wanted to be caught."

"No way," he promptly retorts, fishing a cigarette out of his coat pocket.

"You're so not going to smoke that one, husband dear."

He frowns at the term of endearment as much as at the warning. "Are you seriously expecting me to behave just because you ask?"

"No. I expect you to behave because I have plans for tonight."

His indifferent façade would fool anyone but her. Slowly he puts the cigarette back into his pocket, then checks his phone for incoming texts.

"I still need to get nicotine patches. I've run out of mine."

"Only if you sign all the postcards."

He rolls his eyes and snatches the pen out of her hand.