Telescreen in the antique store

So while Julia and Winston were looking through the window at the singing woman someone speaks to them through a hidden telescreen.
How did they not know that the telescreen was there before?

It's been a while since I read it, but I believe that the telescreen was hidden by a picture that was hanging on the wall.

There was a picture over the telescreen which they just assumed was a normal picture, and I'm guessing the telescreen did not make any kind of noise, unlike the other telescreens.

Winston and Julia did not know the telescreen was in the room they were renting above the antique store because there was an old picture of St. Clement's Church covering the telescreen. The telescreen never spoke or made any type of noise before this event occurred.