About this time of year, back in 1996, I was living next to what had, until quite recently, been a USAF base in the UK. Late one night I was out in the rear garden and saw two white lights above the trees in the direction of the old base. These two lights approached and eventually I could make out that they were on the wingtips of something quite large and kite-shaped. Whatever it was flew over my head, over my house and continued in a straight line, visible for a good ten minutes. It wasn't really possible to get an idea of scale but it reminded me of standing under a B-52 on finals. There was no sound at all. I had always assumed that it was some US black project and, perhaps, the pilot had come to have a look at where he'd previously been based.
The link to the declassified documents is the first time I've ever seen anything that looked remotely similar. Figure 8 on page 9-10 shows a "stealthy ucav concent" which, I assume, should read concept. It's the right sort of shape but I have no idea about size or whether this was actually built. Fascinating read, though.