Doctor Who Series 10: 11 Easter Eggs & References You Might've Missed In 'Thin Ice'
1. Knock Knock
At the end of the episode, Nardole is seen guarding the mysterious vault hidden at St Luke's University when whatever it is on the other side of the door starts to knock. For a while, the prisoner knocks three times but on the final occasion it knocks four times.
This action will immediately bring to mind the Master, specifically John Simm's version who suffered from the constant sound of drums in his head. Seeing as we know that Simm will return later this season, it is looking likely than he's awaiting behind that suspiciously Gallifreyan-looking door.
It's also worth mentioning that next week's episode is also called Knock Knock, though who knows how much it will have to do with what's inside the Vault.