Warning: Spoiler Alert
I'm not sure why it would make me feel happy to know that Lanie and Esposito are still at least somewhat of an item (even if they aren't exclusive, which I would love for them to be). Maybe it has to do with the fact that them still being at least somewhat together means that they are still at least somewhat together ... and there's maybe a little bit of a chance that they might get back together completely. I would love for that to happen, since I love the two of them being together (they make such an awesome couple, and I totally ship them).
But the fact that the fake Lanie had the exact tattoo that she had, in the exact place that she had it (and not that many people knew that she had it in the first place) gives me all kinds of the creeps. How would 3XK have known that she had it? Did he get to someone would have known (maybe the tattoo artist, or someone else that Lanie is seeing)? Or did he have someone sneak in and make the moves on her so that he could get as much info about her as he could? There has to be something there, seeing as how not that many people knew that the tat was there in the first place.
And the fact that Lanie and Esposito are still somewhat together makes me wonder if 3XK was consciously using their relationship to his advantage ... I would imagine so. And now, he's done things against everyone on the team (going after Beckett and Castle first, then Ryan, and now Lanie and Esposito), and he's made it personal for all of them ... and I imagine that he will make it even more personal for all of them before he's finally caught completely.
I think that it was pretty smart of him to have people pretend to be part of the gang on this so that he could get ahold of the information that they had on him; it will make it that much harder for them to be able to be able to catch him later on, but I have a feeling that if he continues to make it personal for them (and continues to rub their noses in the fact that he's still alive, and they haven't caught him), it will make it that much harder for him to remain free (since he will be the bee in their bonnets, so to speak, and they'll want to make sure that they catch him and get him back in prison ... but for a good long time this time, making it much harder for him to get out).