What elements of the setting show that it is a hiding place?

From the play:

The rooms are sparsely furnished, with a few chairs, cots, a table or two. The windows are painted over or covered with makeshift blackout curtains.

Directly under this room is a small, steep stairwell, with steps leading down to a door. This is the only entrance from the building below. When the door is opened, we see that it has been concealed on the outer side by a bookcase attached to it.