Have you ever seen a ghost?

I honestly cannot explain what I saw, but I'd like to think it was some sort of spirit (or at least a spirit did something that caused me to see what I saw). This was about... 6 or 7 years ago, at my grandparent's house. They own a two-floor house, they live in the first floor, and my uncle lives in the second floor (you could say it is two house stacked on top of each other). My grandmother has never done any research to prove this, but she told me once that when she and my grandfather bought the land, they were told that it once used to be a burial ground of the natives (this was in Guatemala, Central America). She told me that many paranormal things occurred since they lived there. My grandmother would see shadows, hear noises, and when my aunts were babies and lived there they would point at things that were not there and be terrorized of their rooms. Why... no one knows. However, the real story is that it was my uncle's birthday. There was a party in the second floor, and no one was in the first floor. I was tired, and didn't want to be around people so I decided to go to the second floor. I went to the back door, and through the translucent window (not completely clear, you could only see the outline of things) I saw what looked like a man wearing white standing about perhaps three feet from the door. I thought it was my grandad, so I waved hi, and asked him to open the door. The.. thing.. didn't move. I went to the front door thinking it might be open, but it was locked. I went back upstairs just to see my grandad sitting with my uncle and my dad. I asked who was downstairs and they said nobody. Being that this was in a slightly dangerous neighborhood they rushed downstairs to see if someone broke in... No one was there and all the doors were locked. Just before I had gone downstairs and saw the thing my grandma had gone to talk to my grandad and told her she saw a shadow moving around by the back of the house... To this day I still want to know what it was that i saw. It honestly looked like a man in white was just standing there! i tried to recreate the moment many times and put things on the other side that might have been there but never was able to see what I saw again.