7 Months Ago, Game Guru says (in Eidos Montreal To Announce Second Project on May 11)  India
The studio said that their second game began with the letter 'T' leading to the conclusion that this new project would either be another installment of the Tomb Raider or Thief franchises. Gamespot reports that according to an ad in Edge magazine, ...

And CVG Online says (in Thief 4 unveiled next week?) The company's website mentions that its internal projects utilise "quality licenses that offer new challenges," and it's already said in an interview that the second game uses an "existing IP" owned by Eidos. We doubt it's Tomb Raider. ...
Thief 4 stepping out of shadows May 11? GameSpot