Circus Gallery (referred to HandUnit as "Circus Baby's Auditorium") is a location in Five Nights at Freddy's: Sister Location within Circus Baby's Entertainment and Rental. It is where Circus Baby and the BidyBabs are located. It is also one of the 3 places seen in the teaser trailer.
Circus Gallery is the only room in the game that cannot be seen or entered into.
Circus Baby and the Bidybabs are never seen in the Circus Gallery even if HandUnit says so.
There seems to be no vents leading to the Circus Gallery, so it is unknown how the BidyBabs get into the Circus Control to try to jumpscare the player.
Whenever the player checks on the Circus Gallery, the lights on the left and right of the windows do not turn on. HandUnit mentions that this will be fixed, but they never seem to get fixed in the game.
Circus Gallery dark as seen in the trailer from Circus Control.