Warning: spoilers ahead for Episode 12 of Arrow Season 5.
Flashbacks have always been a staple of Arrow, and it often takes a while for the action of the past to have any relevance to the action in the present. Flashback Oliver has been getting into Bratva trouble in the Season 5 blasts from the past, and though we hadn't seen too many connections between the past and present yet, the twelfth episode of Season 5 gave us a crazy connection between the timelines that few of us would have guessed. Susan Williams finally pieced together two of the biggest Oliver Queen puzzles... and it was mostly thanks to his tattoo.
Susan Williams has actually been hard at work for a while trying to figure out what Oliver was up to during his five years of hell. She deduced pretty quickly that Oliver wasn't actually marooned on Lian Yu for half a decade, and her private investigator has clearly helped to fill in a few details of Oliver's actions away from the island. She learned that Oliver had some pretty shady connections to the mob, but she had no way of knowing if Oliver was a member of the brotherhood himself. As far as she knew, he could have just been pals with members of the Russian mafia.
That is, as far as she knew until she got a very up close and personal look at Oliver's tattoo, and she gathered enough from her research into the organization to figure out that the tattoo is actually a mark of his initiation into the Bratva brotherhood. While Oliver was cuddling up to the woman he believes is his new doting lady love, Susan was putting together a puzzle that could ruin him. At least one of them was thinking with their upstairs brain.
The Bratva secret wasn't the only one Susan uncovered from under the covers. Her private investigator dug up some old photos from Russia five years ago that happened to show a guy wearing the first Hood suit. Knowing that Oliver was a member of the Bratva in the same timeframe was enough for Susan to connect the dots to Oliver as the Green Arrow. She now knows that Oliver was not on Lian Yu for all five years, he became a member of the Russian mob, and he returned to Star City to take up the mantle of the Emerald Archer. Way to go, Oliver. This is why you don't have nice things.
In hindsight, maybe Oliver should have gotten rid of the Bratva tattoo instead of Slade's dragon tattoo. He still would have had plenty of 'splaining to do to anybody who saw him shirtless, but at least he wouldn't have to try and explain away the mark of the Russian mob on his chest. We'll have to wait and see how Oliver reacts when he inevitably learns that his secret is out.
Personally, I kind of hope that Susan Williams causes all sorts of trouble for Oliver now that she has figured out some of his biggest secrets. He ignored security precautions as the Green Arrow, conflicts of interest as the mayor, and warnings from his smartypants baby sister just so he could date Susan. He needs to see the error of his ways, and if that involves the possible leak of his secret identity leaking to the public, so be it. Stephen Amell even told CinemaBlend that he'd be on board with Oliver admitting that he's the Green Arrow; maybe this is how Arrow will let the secret out.