Aqua steps past Skulduggery to guide him to her room. It's not a far walk - she stays in the room above the stairs - so very quickly she's arrived at the door. Before entering, she turns back to the detective with a somewhat apologetic smile. "Could I ask you to wait out here for a moment? I'd like to make sure she's up to answering questions first."

It's unspoken, but there's also the implication that Aqua wants to give Xion a little warning as to Skulduggery's appearance. If anything, that's what the silent apology is for.

She knocks before opening the door, peering inside. "Xion, are you doing well?"