Found out my boyfriend's ex-girlfriend is my close work unnie.

I'm posting here because this board gets a lot of traffic. Please understand even if it doesn't fit the category... I've been in an office romance for a year now, but we've been keeping it a secret because I hate being the subject of gossip (even my close work unnie doesn't know). My boyfriend, the unnie, and I all know each other—we've even gone out for drinks together a few times. He knows that she and I are absolute besties. But during a random conversation, he let it slip that he had dated her for a year before me? Apparently, they kept it a secret back then too because they didn't want people talking. So I had no idea at all.