Natural, rare atmospheric phenomenon, man-made aircraft and wildlife pretty much explains away most sightings.
The high probability of life in the Goldilocks zone of habitable planets is far exceeded by the extremely low probability of viable travel from those regions of space.
Even if the travel factor is worked out, the likely hood of a visitation to our particular planet even once is remote much less multiple times..
Due to the expansion of space and thus time, even a validated extraterrestrial signal would have been thousands of years old and their society long since gone.
I hate to be the buzzkill. All above I have read.