Cut a 2 foot square hole in the wall, 8 feet from the floor. Use the cut piece as a door, swung out from the wall. The wall has to be deep & dark enough that you don't notice the back side. Inside the wall, 3D sound and a rotating light suggest a pulsar.
An alternative would be something at baseboard height, throbbing and steaming, as if something Crawled Out of the Woodwork.