Undiscovered natural phenomenon - As was Sprite lightning, which was documented for years, but only first photographed in 1994. In 2000 the UK MOD concluded in the Project Condign report that UFOs were "indisputable" and that many of these objects could be a plasma based energy with the ability to adversely affect technology and people who come in contact with it. Also see the "Hessdalen lights" in Norway.
An artefact / insert / bug of a simulation theory universe
Cryptoterestrial - An undiscovered species or intelligence we share the planet with
Interdimensional - Artefact from one of the 26 dimensions of string theory
Extratempestrial - Something from an overlapping time
I think you need to get away from thinking there are little men in ships and think more about how an AI or undiscovered life form might operate in the universe.
I understand why all of this seems completely bat **** mental to an onlooker. The problem with this phenomenon is that it is not something that can be easily studied with the scientific method. How do you create lab conditions and repeatability for testing? What I would say is take note of what the US Congress are doing, what NASA are doing, what Nobel Prize nominated Garry Nolan is doing and what AARO the DOD agency setup last month to uncover the source of this phenomenon are doing.
The Defense Department announced the establishment of the All-domain Anomaly Resolution Office.
www.defense.gov
Disclosure that a phenomenon exists has already happened. It is going to take people time to get their head around it and that is understandable, but to brush it off is a mistake. If you want to be ahead of the curve look into it because this topic isn't going away.