When they try and fool ianthe that nona is harrow for a second ianthe gets suspicious and seems to worry its gideon? But she clearly knows gideon is undead and well in another room. So whats that about is she worried gideons soul is split in two or about something else?
It does seem like she worried it's someone or something not Harrow.
She might think it's Gideon. She knows Kiriona is chilling in her room, but Ianthe's body is alive and well in orbit while she walks around in Babs. Harrow and Gideon are one flesh, so if no one else was home in Harrow's body it's possible Gideon might be able to toggle between them.
Or maybe she thinks Camilla found some way to funnel Palamedes into Harrow's empty body after Gideon got yoinked out. Not impossible, he does fight Ianthe for Babs later. I'd wonder if she thinks it's Number Seven, but if she thought that I think she would have reacted differently to Nona's scream.
Most likely she's fishing. She doesn't finish the sentence because she suspects it's not Harrow, but doesn't actually know who else it could be. She's hoping if she acts like she already knows, Nona will just tell her.