My dad was a very intelligent, wise and good honest man, I want to stress this to give you a little of what he was like so some of you don't think this is a load of rubbish
Even intelligent, wise, honest men can make mistakes, sugar.
Our eyes can't actually "see" anything as such - they merely collect light & convert it into electrical impulses. Our brains use that information to construct an image for us. Sometimes - quite often, actually - we "see" something that our brains can't make immediate sense of so it constructs an image in our minds. If we don't have the opportunity to go back and ascertain properly what we actually saw, then the image of what we thought it was remains.
I don't doubt your Dad saw something, it may even have been reminiscent of a human shape - but it could have been anything. It could even have actually been a woman/man jumping off the wall but who made a hasty retreat without your Dad seeing - a burglar?
In any event, the fact is that any and all rational explanations that don't appeal to the supernatural are, literally, trillions of times more likely than that the spirit of a dead person jumped off a wall in front of a car.
And, sorry, who exactly saw your Grandad sitting in a chair at the same time he was in the kitchen? From what you've said, it doesn't sound like it was either your Dad or Grandad - a neighbour passing the window?
You seem to think (as most people do) that anyone trying to present a rational explanation thinks that your Dad must be a deluded liar. Not so. Just a mistaken one.
Indigo Yes, eyes are organs that collect light that the brain can use to create an image. Video cameras do the same thing. Anything you can see, so can a video camera. This is simple physics.
Of course, the one thing a camera can't pick up is what is going on in your imagination, which is clearly where these walking dead people you can see reside.
We can still see images when we shut our eyes because our eyelids are not made of steel - some light filters through. And we can see imaginary images because our brain does not have to have the direct input of light to be able to construct an image.
Tell you what - why don't you tell your ghost friends to go and talk to a scientist? Odd that they never do that - are they scared of scientists? It's the very first thing I'd do if I found that I'd somehow survived my own death.
I haven't taken anything personally. I don't believe you are being truthful - and if I genuinely thought you could actually see ghosts walking next to you I'd very concerned for your mental health. Really.