The unit you are looking for, is going to need power.
Start by looking at all the electrical outlets between where the main equipment is and the TVs. See if you can find a cord plugged in (or wallwart) and a wire that disappears into the wall or ceiling. The direction of that wire will at least let you know if you should be looking in the attic or under the house and the general area.
Don't forget to check the top shelf in any closets along the way, where personal clutter maybe hiding wires going in & out.

All else fails, someone might need to search every inch of the crawl space or attic to find. Might find a junction box that someone hardwired the electrical for the little black box and scotch locks on everything lol