The Basement
(Concept not final, please discuss)
While the IKEA might be infinite, the basement is not. You need a key card from a Staff to get in.
Once you do, you realize that the basement is actually a pocket dimension and is quite small. The inside has only 1 door, despite being accessible from several locations. The door only opens for 1 of these at a time, however. When a person open the door, they can return to the opening they came from, or walk through into the area a different person came from.