You would not have seen the buses leaving the US, only when returning, since customs is only required upon return. Flights go to Europe from Dulles or BWI, but Belgium would likely be just Dulles.
Speaking of Dulles, if you happen to fly into or out of the airport, the Air and Space Museum Annex is very close to the airport, and would be a great way to spend a few hours waiting for a plane.
Jim Bede had to stop making airplanes for 10 years because of a consent decree with the FTC related to the fact that he was a truly craptastic businessman and his customers gave him money and didn’t get airplanes. So rather than get a normal job, he used the molds for the BD-5 airplane to make the Litestar Pulse 3-wheeler (which had 4 wheels! but only 3 touched at any one time).