My grandfather (dads father) died when I was about 5, family tell me he was particularly fond of me, more so than his other grand kids.
When I was about 20, newly married and living in a small rented flat I saw him about 5 times watching me. Two times I was in a position to grab and talk but failed. 1st time I was in a small bathroom shaving and he was behind me watching (saw him in mirror), I was slightly closer to the door than him so like a flash cut off any exit from the room, swung around and the room was empty. 2nd time I had that feeling I was being followed on the street, swung around and he was about 5 mtr behind, he turned and ran around a street corner, I chased, turned the corner and the street was empty, no door ways or street exit points within 10 or 15 mtrs.
Have a couple of unexplained lights in the sky stories but people are always happy to put arguments up and I'm not in any mood to argue.
My grandfather story is firsthand, personal and no drugs were involved.