[awesome name] Opera House Dressing Rooms

The [something] Collective had agreed to make a small expansion - just one building, small, unassuming, but seemingly repurposeable - years ago. As with any expansion, they began by building a tunnel to connect it to another building within their network. This was customary to create an invisible means of entering a building to avoid the suspicion of [place's inhabitants] at the sudden activity in a new building. They eagerly began building, hoping to create a livable house for their expanding secret community. Upon inspection, however, they discovered that this small house was hardly functional. It was filled with rooms that were mirrored on every wall, halls with abrupt dead-ends, doorways that have been long-since boarded up and sealed, and yet in all its varied rooms, not a single kitchen or bedroom to be found. The building, however was cool and dark so, clinging on to hope, they decided instead to turn it into a fortified storehouse for their preserved foods. They boarded up all the windows and exits they could find, yet they could not find the source of or stop the tide of small rodents who would sneak in and feast upon their precious stores. Finding the restoration and upkeep more work than it was worth, the Collective withdrew from this property and locked the entrance to its tunnel.
The building remained undisturbed for the first decade of [character]'s life, until they discovered they had an insatiable curiosity and a knack for picking locks. In secret they made visits to the twice abandoned building over the years, poking around in all its corners, discovering its every nook and cranny. At first [character] would easily get lost, to rectify this they began plotting the structure's dimensions on a map they kept hidden in the tunnel. In their rare visits into the no-man's-land outside the [collective] network, they stole glimpses of the building's exterior and noticed an extrusion that didn't match the shape of its interior. Curiosity at an all time high, they began exploring the building with more skepticism. Knocking on walls, scrutinizing the dead-ends, even pulling up loose floorboards. Long at last, they discovered a tiny room whose door had been walled over. Though the room was bare and its windows similarly sealed over, the paint on the walls had faded, revealing the shapes of the life that had once existed here. It was with these outlines that [character] discovered the entrance to another tunnel, created long before the Collective came into existence.
At the end of this tunnel lie a great room with high ceiling, a giant arch, and a wooden platform overlooking rows and rows of seats. The abandoned building had once served as the auxillary dressing rooms of [character]'s newest discovery, the [awesome name] Opera House.