enormous triangle gliding silently overhead

This sighting occurred during one of the worst air inversions ever in the Salt Lake valley. The inversion was so thick that vehicles had their headlights on at noon.
I was riding in the bed of a pickup truck enjoying the reflection of the city lights on the dense smog/fog layer above me. 
Suddenly, an enormous triangle glided silently over my head. I would guess it to be 3-4 times larger than a commercial airliner. It was 50 feet above me and was dark gray. It had no lights and the reflection of the city lights revealed the gray color.
I stood up and watched it glide into the night. the whole experience was 20 seconds at best.