Hi - I thought I bookmarked this, but obviously I didn't.

It was posted sometime in 2011, and it was a claiming/mates sort of fic where EVERYONE had, somewhere, a mate. Jensen was a writer that was unmated and he was old enough that he was declared, or was about to be declared, "Widowed but Unmated" - meaning he hadn't found his mate, so his mate was presumed to be dead.

Jared was a college student and a big fan of Jensen's. He and his usual crowd of friends (Chad, Sandy, etc) were in line at a booksigning for Jensen's latest book when their eyes meet and they recognize each other as mates. Semi-public sex ensues.

ETA: FOUND! By katbcoll - Not Aloneby the_miss_lv

Also suggested: Lost and Found by motherrussia91, thanks to sinestrated - it was written from the same prompt, but was not the story I had been looking for.