Most likely, they are coming from whichever ground radar is picking them up. Around here, you have a ton of possibilities... Potomac TRACON, Andrews AFB, Langley AFB, etc... also, it could be air-air if someone (either military or civilian) actually saw someone else enter the zone. There is a HUGE amount of restricted airspace in a 50-mile radius of DC. Almost all of DC, pockets up to Frederick, most of Baltimore, anywhere near DCA, IAD, BWI have restrictions. Also, in southern MD you've got Pax River and Calvert Cliffs NFZ's.