I don't think that is it, the ceilings were way too high for what would almost certainly have been a drizzle patch.
I went back to look at the 200 loop CoD radar image and I'm stumped. There are two areas, one starting in western Baltimore and another in Tacoma Park. They drift SSE which does align with the wind direction. If you look at the polarimetric parameters it clearly isn't precipitation, but you'd expect a different Zdr if it was birds (migration was my first guess).
It also isn't vehicles. Those show up much sharper on radar right along the roadway when there is a good inversion. This became more diffuse.
My guess is still biological, but what I'm not sure.