So I went out last night in search of them around 7:50pm. Popped in at ANS
(there is a smallish field near the entrance), and some fields nearby on
Jones Mill Road... nothing.  Then drove down Beach Drive, over to Oregon and
to the  big field at Rock Creek Park, near Military Drive... nada.

Next place to try - the National Arboretum - it has a nice big field with
woods nearby.

Huntley Meadows and Sycamore Landing are the closest so far. 

Cheers
Tim