This is going back nearly 20 years, but I will never forget it. It was the summer of 1996 or 1997. It was a warm evening, and my best friend (male, age 15/16) and I (female, age 15/16) were lying on our backs on his backyard trampoline, looking up at the stars and talking. As we talked, I noticed what I assumed to be a satellite -- a small, star-like object traveling silently at a constant, slow rate of speed across the night sky. I pointed it out to him, and he saw it, too. Then we noticed another identical one nearby traveling at the same rate of speed. "Wow, two of them, what are the odds?" was our reaction, until we quickly noticed a third identical light, and that all three lights were moving in formation as a perfect equilateral triangle.
Just as we were commenting on how weird that was, we noticed that as the three lights passed across the sky, the "triangle" they formed blocked our view of the stars between them. That's when we realized it was a single solid object, not three satellites. We both became terrified and ran inside his house to get away from it.
I have no idea how high up it was, but since the lights looked like little satellites, I think it was pretty high up. If so, it had to be huge -- maybe hundreds of feet across. It made no sound and was traveling North-by-Northeast in a straight line at a constant, slow rate of speed. I think that if we had stayed outside to watch it, it could have taken at least five or ten minutes to pass out of our range of view. We lived right next to Vandenberg AFB, and in the years since the incident, I have come to believe that what we saw was an advanced, "skunk works"-type aircraft on a test flight. At the time, however, it scared the crap out of us.