Indiana: "Uncle Jack, don't worry about Nate and Lola being at our sex party. We have bigger secrets, like that secret person we're keeping in this sex house where the entire cast just showed up."
Understandably, there are a million cameras all over this sex mansion, so it's of course at this precise moment that she sees Chuck and Blair touring the entire place. The one thing that could wreck her ludicrous plan to conceal Mr. Rochester's First Wife or whoever it is while also throwing a sex party under her double-identity of India, the Madam Nobody Ever Recognizes.
Chair: "If we were going to fuck in a dumbwaiter, that might be interesting."
Serena: "You guys! Why are you here at this sex party!?"
Everybody: "Why are any of us here at this sex party?"
Serena: "I'm here tracking Gossip Girl's burner phone. I presume you're here tracking Indiana and Uncle-Father Jack?"
Everybody: "Well, it was nice running into you. I hope nobody sees a single one of us and makes the obvious connection to the rest of us..."
Police Raid! Just as Serena is apparently using her spy satellites to locate Gossip Girl somewhere in the sex mansion. Oh, and you were so close, girl. So ridiculously, unbelievably close. More believable: She's the only one of the castmembers who actually ends up leaving during the fake police raid, and she doesn't run into Dan out there. Even Nate and Lola manage to sneak up the stairs, discovering Indiana in one room, while Chuck is finding Jack in a second room, and a curious thing is happening in a third room.
Blair, affectingly enough: "Oh my God, no. No, it can't be..."
Who is Rochester's Wife? Who is up there? Elizabeth Fischer wouldn't scare her like this. Eric and Jenny are never coming back. And she's also super-sad, too. It's neither of Chuck's parents-of-the-moment, because they're in the other two rooms. So unless it's somebody unexpected having some scary sex, which it is not, it's going to be ... Bart, right? God damn it. Just goddammit. Ed Westwick is a good actor! (Maybe technically a great one, if you consider how many constraints playing Chuck Bass actually entails.) Bart's death was one of his greatest things. So rather than giving Ed Westwick something to work with, let's by all means start retroactively taking previous things away. That makes sense, right?
Jack: "Chuck, I'd love to chat, but..."