I'll let you know at the end exactly when this story takes place, but for now have fun with Leonard and figure out what's going on on your lonesome. IC and IAW canon, as always.

Neckware and Semiotics for the Advanced Beginner: Chapter One

Leonard stared dumbfounded at the tie on his apartment door. It was certainly Sheldon's tie; He recognized the pattern vividly from the night Sheldon dropped his pants after imbibing just a bit too much liquid courage. And Sheldon was the only other person who lived there...

Unless you counted Penny. And you could, since once they'd started sleeping together again last month, the two of them went back and forth so much you could consider the whole floor a three bedroom suite. But he couldn't figure out what kind of reason she would have for hanging it there either. While they did engage in a little hanky-panky in the living room… or the kitchen… from time to time, it was certainly never preplanned. Anyway he know for a fact she was working a bar shift at the Cheesecake Factory right now, and although Leonard had originally planned on browsing used video games with the other guys that afternoon, he'd ducked out when Howard and Stuart got into it again over who should go with Raj to The Rocky Horror Picture Show next week.

Sheldon was supposed to be home. But the tie still made no more sense than it had five minutes ago when he froze in the hallway with his hand two inches from the doorknob. It wasn't the first time he'd seen something like this. His roommate in college had hung one on their door several times, often forgetting to take it off and finding it hilarious to come out in the morning and find Leonard asleep in the hallway. He'd even hung one on his bedroom door in this very apartment once. He didn't know at the time if Sheldon would even know what it meant, and he was half afraid he would open the door to ask, but he was even more certain if he didn't take the chance there would just be a repeat of that awful 'date' he'd had with Joyce Kim. Sheldon ended up calling him anyway to tell him he knew Leslie was in there and the message was received, but looking at this tie on this door at this moment, Leonard's brain flat out refused to call on Occam's Razor to jump to the most likely conclusion.

Did Sheldon really know what it meant?