Probably done testing whatever it was, most likely some kind of tech for drones or maybe even the B-21 or F-47. Just like the "Flying triangles" from back in the 1980's and early 1990's were probably the F-117 and B-2 test articles.
There is several guys that on YouTube that seriously investigate the UFO sightings and almost all of them are either flares, children's balloons or in some cases people literally filming a reflection of a light fitting in a hotel room.