People see things that they don't see on a standard basis. Either that, or they just aren't paying attention to their surroundings.
Apparitions and spectres are nothing more than visual abnormalities you are not used to seeing. And most people that refuse to find any explanation for them, link them to whatever supernatural or extraterrestrial beliefs they have.
I don't believe in ghosts, goblins, or monsters under the bed. But I have seen things that defy explanation...............that is until I research them and find out what they really are.
The world is full of happenings that happen every day, that would seem "weird" or "unnatural" if we care to look at our surroundings every day and break down everything we are seeing. Just because SOME people happen to notice these things once in a while, and relate them to superstitions or witchery, doesn't mean that's what they are.
I believe in ghosts, as much as I believe in sparkly, glowing vampires.