The snowpocalypse of February 2010 was hard on buildings; this is no exception. The structure was in worse shape than in brentbeebe's picture.
(click any image for bigger)
I found a bag of nails under the floor. The bag came from Whole Foods, which has a store a few miles north of here in Rockville. I can't say the bag definitely came from the Rockville Whole Foods store, but that store (originally called Fresh Fields) became Whole Foods in late 1996.
There was a rusty saw tucked into the rafters. I found other objects nearby which appeared to be a sleeping bag and a parka. In the general area were a candle in a vase, an empty bag of "Hotsticks" (a brand of kindling wood), and a black plastic tarp (which was also used in the structure's roof). I did not find many food-related items. There was an empty plastic yogurt cup, but there is plenty of random litter of that type in the area.
The floor joists are viewed from below. Notice the nails (rather poorly) hammered in along the length of some of them. I don't know if the nails actually held anything together. None of the wood was painted or finished in any way. There was no writing, carving, or other marks on any of the wood. The general level of decay suggests that it has been abandoned for several years.
My best guess is that a homeless person built this as a shelter around 2000. I don't know where the wood came from or how it got there, although it was probably carried in. The barriers formed by the Beltway and Rock Creek, as well as the soft ground and many trees would make it difficult to bring in a vehicle. If the parka and sleeping bag came from the structure, it suggests that it was abandoned without removing anything. Perhaps the occupant was arrested, sick, or injured and unable to return.