It appears to have an observer quality to it. The one that sticks out the most in my recent memory is the 2006 Chicago O'Hare sighting. I remember this going through a pretty quick 1 or 2 day news cycle. Fairly alarming given the number of people witnessing the phenomenon and the backgrounds of the people observing it.
https://en.wikipedia.org/wiki/2006_O%27Hare_International_Ai...
Obviously rainbows are an example of a shared hallucination and the phenomena manifests differently dependent on the observer's viewing angle. At the moment I'm not convinced these aren't unexplained atmospheric phenomena because I've never seen one. I know some ideas put forth include ball lightning for at least some UFOs. Assuming these cases to involve intelligence may just be our misinterpretation of the phenomena.