If UFOs exist, ghosts materialize to haunt places, or even such things as Christianity's angels and devils exist could be explained by the idea that there are multiple dimensions that intersect, and under normal conditions no visibility, communication or direct interaction can happen between these dimensions. At best, there would be be only rare moments when something aligns to reveal a fleeting connection between two or more dimensions. Or if you'd prefer an advanced technology in one dimension might have the know how to create a stable connection.

• UFOs might just be flying machines from another dimension which have temporarily revealed themself knowingly or not in our airspace.
• Angels, Demons, Fairies, Fey, Ghosts, and all things that spawn folklore and fantasy since the dawn of time could simply be other societies living on another plain of existence, aka another dimension. That's sort of how many a great novel has been written to explain that which is alien but also not.

Imagine if you will a crow because they have Tetrachromatic Sight. Compare the crow against Humans who are trichromatic and only see red, green, and blue. Crows and most birds have a fourth cone receptor sensitive to ultraviolet or violet light therefor they see things that humans are blind to. So to a crow there is a lot more to be seen if they simply look around, whereas humans are blind to these things. If there was a being, a virus, technology or anything for example that required another color spectrum or sense to spot, it would be as good as being invisible to humans by comparison. Imagine if you will a creature or even robot that has technology to cloak itself in something that humans can't perceive. Then imagine throwing a glitter bomb and hitting. You'd see the glitter, outlining whatever is there. You'd probably then let your imagination run wild and answer the rest of your questions. Long story short, corvids have less questions since they can perceive more.