In the first part of Inception, Cobb’s crew leave the dream where they attempted to extract Saito’s secrets in the castle, and return to a dream (though this is unknown to Saito at this point) in Saito’s apartment.
Saito remarks that nobody else knows about this place, and soon realizes that he is still dreaming when he is forced onto a carpet, by Cobb, which is made of a different material to the one in the apartment he uses in real life.
After leaving this dream, Arthur asks Nash (the architect of the apartment dream) how he could have screwed this detail up, and Nash says that he didn’t know Cobb was going to rub his cheek on it.
If nobody else knew about Saito’s apartment, how could Nash have created it in such almost perfect detail within the dream?
Did he simply observe Saito in the apartment in real life, or is there some other mechanic going on that Nash could perhaps extract the information from Saito in the castle dream and use it to create the apartment environment?