At 2:20, a man in a green shirt and woman walk behind Niles. Then the camera focuses on Frasier and Nancy, and back to Niles where the same couple is walking behind him again.
As Frasier goes to the phone to call Nancy, wheels can be seen on the bottom of the piano legs. This is a tell-tale sign that the sets, however consistent in design, are not permanent. Frasier would never leave the wheels on after the piano was moved into his apartment, especially on his hardwood floors.