Various existing and proposed UAVs can be seen on this page.

If any of these were flying as a classified vehicle, like the old F-117, one could be forgiven for thinking they were seeing something weird.

I always thought a plausible theory for giant triangles was the triangular airship concept proposed for stealth observation. Rumor had it that it had the ability to project a night sky on its belly. If so, it could explain how some of these sightings could "vanish."

Of course, that doesn't explain why "secret" aircraft run around with brilliant lights all over them, or who would be idiot enough to fly an airship anywhere near a thunderstorm.