I've been fascinated by ghostly things for most of my life, but I've only personally experienced one event which, to me, only allows a ghostly explanation.
At the time, I was working as a part time librarian at a community college. Part of the campus had been built over an old Roman Catholic cemetery. Though it was moved to make room for the campus, the library building, which had the most apparent activity, was not built on the site of the cemetery.
One night, around 10:30 PM, having closed and locked the library, and with the assistance of two other staff, I checked the building top to bottom and confirmed no one else was present, making it, I thought, the perfect time to do a little light reading. I headed to the top floor to find a certain book, but could hear the distinct sound of typing coming from the computer lab. The computers there had keyboards that made a solid clicking sound when pressed. Figuring someone had managed to sneak around our search to clear the library, I poked into the computer lab to check.
The room was devoid of people save for myself, yet I could clearly see the keys on one keyboard moving as if someone was typing up some homework.
That is the only ghostly event I've witnessed where I could not come up with any alternative explanation.
At this same community college, I experienced one more event. This happened around 2 in the afternoon on a very windy autumn day, so I suspect the blowing leaves had something to do with it. I was standing in a building actually built over the cemetery, talking with a security guard about something when we witnessed the doors to the building open and close on their own four or five times in about 2 minutes. It was an airlock type of door with outer doors and inner doors, both controlled by an optical sensor. I've never witnessed blowing leaves trigger such a door before, but it happened quite a lot with myself and the security guard witnessing it.