I think ghosts are real.
Have you ever indulged in those illusions that have you stare at an inverted image for a long while, then turn to face a blank wall? You are greeted by the negative of the image.
Similarly, when you know someone for a long time, you have seen them for a long time. Not just in form but in action and impact on their surroundings. So, when they abruptly leave, you are left with the negative, they space they once filled in your life now empty.
Couple this with grief, it is completely understandable for people to see long departed loved once still in the places they used to live.
Ghosts are real, because the grief and love that make them up are real.