What Are White Aerial Triangles Zooming in Earth Skies?

— “I was posted as a security guard at Zel and Mary Loman Elementary School about 6 or 7 o’clock in the evening directly across from Nellis Air Force Base, Nevada, and every car alarm for 10 blocks went off on the ground  ... as the white triangle rolled into my vision — it literally did a barrel roll. I saw top and bottom and then it hit some kind of engine boost and disappeared in the blink of an eye.”   - Elementary School Security Guard, Las Vegas, NV

— I had the impression that it knew it was being watched. Then the white triangle stopped and stayed still for about 30 seconds before becoming smaller, I assume it was rising, and then vanishing from sight.   - Resident of Bath, England