I saw something similar late one night in the late 1970's in Oklahoma when I was in the Air Force, except I would describe it as being more shield-shaped rather than triangular. It seemed fairly large, went very slowly from one end of the sky to the other and disappeared in the distance. I even called the tower to see if they saw anything. They said they didn't but I'm still convinced I did. I don't know if it was a UFO or some secret government aircraft, but I saw something.
The “U” means unidentified. I lean toward the Carl Sagan point of view. I’ll believe they are aliens when they show up for the DNA test.
A friend works for AFRL and we saw the Captain America movie where SHIELD had flying aircraft carriers together.
At the dramatic moment where the flying aircraft carriers are shown for the first time he went into giggling fits.
After the movie I asked him what was so funny. He kept waving me off and changing the subject.
Finally, hours later, when we were having dinner, out of the blue he said, “Ya know, they don’t look anything like that at all.”