The Break-in

Around six years ago, there was a break-in at the Democratic party's headquarters during the last election run for Senator Newbold. (At this time, S Mercer was working as his campaign manager.) The Democrats made a big deal about it, claiming that Senator Newbold's campaign was playing dirty tricks and trying to steal campaign information.
What they didn't know was that the campaign had nothing directly to do with it -- instead, the break-in was orchestrated by N Stanimir, who hired a few loyal supporters to do it, specifically looking for dirt on the Democratic candidate, who was passionately anti-gun. What they did find was some internal backgrounders, which revealed that the candidate had an aunt who was an illegal immigrant. They leaked this to V Leonard, who blew it up into a lovely scandal, sinking the Democratic campaign and letting R Newbold sail through to victory.
That said, the intruders weren't exactly professionals: they had made it look like a simply robbery for computer equipment, but a laundry receipt had fallen out of one of their pockets. Fortunately, the Senator had asked his friend the Police Chief to give B Ari a chance at a big case, and Bjorn found the receipt. He looked into that personally on his off hours, tracked down the culprit, and figured out the connection. He quietly went to the Senator and explained what he had found; Reagan went to Simon about it, and Simon chewed Nika out over it.
That caused a little friction for a little while, but the truth is that the Senator was impressed by Stanimir's gumption, and was more annoyed at the fact that the whole thing was sloppy than that it had been done. After the election was over, they got to know each other better, which eventually led to Stanimir becoming Newbold's campaign manager this time around -- Reagan was somewhat turned off by having someone quite so cynical as S Mercer as campaign manager, and wanted someone more sincere next time.
There is only one tiny snag: R Finlay was a beat cop during that investigation, and was helping guard the scene while the investigators looked into it. She was sure she saw Bjorn pick something up, look at it, and stuff it into his pocket. Rhona confronted Bjorn about it back at the station house, and Bjorn claimed (unconvincingly) that nothing had happened and that she was imagining things. That was the beginning of tensions between the two of them, which continue to this day. Rhona mentioned the whole thing to M Caoilinn a few years ago; by that point it was largely water under the bridge, but it has turned into vague rumor ever since.
And of course, the whole thing is sitting in S Mercer's Database of Doom. He feels a little more willing to talk about this particular piece of political stupid, having been uninvolved with the planning, and it seems like a potential way to embarrass the Senator.