How do you explain ghost sightings to others?

I’m sitting in my office yesterday, working at my computer. My desk faces the wall, with the door to the left. Someone peeked into my office from the left side of the door, about 5’6” to 5’8” inches tall, and then pulled back quickly. I always get people stopping in to my office when they get lost, because my door is always open and I play a lot of loud rock and roll. I went to the door and there was no one there in the hallway. I know I saw something looking into my office. It was cream colored, not black or white. What the hell was it? It totally freaked me out. This building is supposed to be haunted, but mainly on the floors above me. I’m 10 to 12 feet from the door, so distance didn’t have anything to do with it.