UFO's or explainable phenomenon?

I personally think there is enough evidence to say ET most likely exists but the universe is a big place. So let's assume there is a civilization of intellegent jelly fish for example on the other side of the Galaxy but a craft is passing near by and they scan earth realizing that yes we have an abundance of life. But our technology and evolution has not yet merrited first contact by an intersteller or intergalactic being so they keep their distance or hide themselves from us. I am sure it would not take them long to realize that humans are a war drivin, greedy, primative race that would try to down their space craft for our own personal gain.

However it is possible and i have seen cases that really defy any reasonable explanation. It is just that these apparently genuine cases are not as common as the not so genuine sketchy cases that may be man made or natural phenomenon like ball lightning.