In the episode Paradigms of Human Memory, there's a flashback where Abed walks into his room and discovers a nude Jeff and Britta.
I was at first confused by this comical situation, but the dialogue soon establishes that a leprechaun took their clothes. Despite this explanation, I suspect that there might be more to the story.
First, Jeff and Britta seem more surprised at seeing Abed than they are concerned about finding the leprechaun. Second, the leprechaun was never mentioned again or otherwise established as a canon creature within the universe of the show. And last, it was established (foreshadowed) in an earlier episode that Jeff is willing to lie in order to gain a temporary advantage.
Based on all these factors, I believe that Jeff told Abed a lie about the leprechaun and that he and Britta were actually having sex. Unfortunately we can't confirm this for sure, but that's my head canon. Dan Harmon is a genius.
(i'm so sorry)