#something's off

A nice subtle bit of acting from David Tennant where you can see the slight shift in the Doctor's eyes, as specified in the script. This leans into the character's intelligence: he can already tell that something's up with Rose but quietly keeps it to himself, even tacking on a perfunctory "You're right, well done" to show he's playing along.

This is not unlike an earlier scene with Novice Hame telling him the story of the Face of Boe - the Doctor silently recognizes himself in the "Lonely God" description, but doesn't say anything. Watch that slight pause he gives here, right before casually asking Rose/Cassandra about the subframe: it's been established previously that Rose isn't all that well-versed in sci-fi technobabble, so the Doctor's question is a deliberate act of ignorance; a test to confirm whether his suspicions are correct - since of course, Rose herself 100% wouldn't have known the answer to that...

We see the terminal very briefly before this. Earlier in the episode, the Doctor exits the disinfectant lift and the camera follows him as he ruffles up his hair again... straight past the terminal display we see him and Rose examining later.

When Cassandra/Rose asks, "Why would they hide a whole department?" there's a tiny instance of her breaking off almost mid-sentence to visibly run her tongue around her teeth. A visual callback to the very first thing the Tenth Doctor said, back in The Parting of the Ways, maybe? "New teeth... that's weird." If so, it's a clever acting choice by Billie Piper: as though Cassandra is still getting used to her new mouth (which she used quite extensively in the last scene!).

In fact, Billie is great throughout this - bringing the character's arrogance to the fore, so intent on her own motives that she doesn't pick up at all on the Doctor's suspicion. There's a lot of aggressive tension Billie has around her mouth during this scene; watch the predatory way she sort of curls her lip in satisfaction right before marching into Intensive Care.

And of course, she walks in first. Way too confident for Rose.

The way Cassandra drawls out the sentence "Nurse... cat... nuns": possibly echoing to the "Old Earth saying" she recited to Chip when she was fixing herself up in front of the mirror?