Big thanks to Dragons-And-Merlins-Beard for doing proofreading for me!
The hat detective was not a fan of the 14th of February. It was merely one of those days among 365 days to him. It was, however, different this year and he was staring at a greeting card arrived few hours ago.
The card was not sent through the royal mail, and the mail box of 221B is where it delivered to. There was only "Sherlock Holmes" written on the envelope; big, pink "Happy Valentine's Day" printed in cursive font on the front of the card, and a hand-drawn smiley inside of it, unsigned. It came apparently from Moriarty, who paid a visit last month.
Sherlock checked the materials, weight, bar code and serial numbers of the card and ran a mind scan with the possible code tables. Every time when he thought he almost cracked the hidden message, there were always one digit or two failing to match, messing with his reading. He left the card on the experiment table, so it would go out of sight, out of mind. From time to time, his eyes couldn't help but linger on it, even John sensed Sherlock's troubled mind. Sherlock excused his distraction by an unsolved trivial case.
Sherlock spilled the cup in an experiment at night and water ran over the card. He dried it with an alcohol burner at hand and found the words previously unseen appeared-
"Invisible ink is so 19th century but efficient enough to trap you. JM x"
Sherlock took his phone and texted a message back in response immediately,
"Thanks. Head over heels for your card. ＠}- SH"
Moriarty didn't reply, but Sherlock knew he would read it somewhere in the UK with a smug grin.
It didn't occur to Sherlock that his heart would suspend all day just for a card and Valentine's Day was somehow more intriguing than he had thought before. He read the card again, placed it in a book with a gentle smile emerging on his face.