I don't know if we will ever understand what makes a ghost visible to us or how it is created. There seems to be an an ability factor from the person seeing them, and nearly always there is fear involved by the person who is the ghost.
But I seem to be able to read objects as well as places that have no soul per se, even without involving people as well. Unless you believe everything has a soul. It certainly seems to have a memory.
I remember walking into a building I have never been to before. (It was Newmains where I stayed for week and was very active). I looked to the right in the hall and briefly saw a long case grandfather clock - but there was not one there. After exploring the house more I saw the clock which was upstairs on the first floor. I went back down to the hall and had a closer look at where I had seen it, and there were pressure marks in the carpet just about visible that matched the base of the clock upstairs - so it had been there in the past. But how could I know that?
I've also experienced a cellar that I could feel lines of shelves in as I walked around, but it was now open plan, but in my minds eye I could see these wooden shelves full of items in there.
At 22 Belmont street Aberdeen I have seen a room where there is none today, and a window behind me that is not there, and outside steps from a rear door that are not there today, but all existed in 1850 and is shown on the maps.
At the docks I have seen a warehouse from the inside, but the building was demolished and a concrete car park built on the spot in the 60's. Old photographs show that there was a warehouse there built in 1700 and matched what I saw with the narrow slots for windows.
So it is not just people that leave ghosts behind to be seen, but places and objects too. Maybe what we know as ghosts are not even there at all, and it is just an ability of the human spirit to transcend time to see these things. We just need a marker as a reference to stop and look around in that time like flicking through a book.