looks like one of those terminators they use on a multi block setup to prevent folks from tapping into the box and get cable.
At the end of our townhouse row there is the cable junction box. If you take the cover off (it looks like a Rubbermaid box) there is 8 outputs for 8 townhouses. 4 folks have cable and the others dont so they put those items on the unused ports to prevent folks just hooking up cable and getting it for free. They require a special tool to take off.