Was it a green box pointed in the air?
There are several of those batteries in and surrounding DC. I run boat cruises for private groups and you can see one from the water. There is definitely one at JBAB. I'm sure there a longer ranged SAM battery somewhere too.
I heard a few buildings in DC also have anti air arty (think guns and cannons) on the roof of some buildings as well.
Aside from the surface to air missiles, I believe we only have a battalion of immediate armed troops to defend DC. Having grown up in South Korea, this seems to be a really low number (they have divisions of soldiers defending their capital). But then again our (US) enemies have to cross huge oceans to get here.

Anyways, most of this knowledge is from my guests who were in the military or were contractors involved in these projects so please correct me if I'm wrong. Please be cognizant of the fact that even unclassified information being shared on the internet is frowned upon if it is detailed enough..