The following are couple of activities are are ghostly in nature but are not actually ghosts. If this happen to you when it is middle of the night and you are home alone, you will probably freak out to death and run. But these are actually not ghosts. I am sure some of these you have seen in ghost stories.
Moving Pad Lock
I lock the padlock that we use to lock the main gate at night. After a minute or two, I noticed the pad lock was moving back and forth. That was really freaking any ghostly. But since I was not not home alone, I did not scare that much (it did scare me though). Since I lock the door only less than a minute ago, I figured it was just because of that. Later I analyzed that lock and noticed that it will oscillate for about 5 minutes or so once you move.
Ghostly scenario could be, there was a wind or somebody touched it etc that cause it to oscillate at night and that makes a perfect ghostly activity.
Running Water of Tab
If this happened to you at night and you heard bout ghost in that room, you will probably freak out. But it happened to me that tab water continues without any human interaction. This happened in the day. If you close the tab, at one point it starts to flow automatically (may be loosens up by itself). Again this would be perfect ghosts at night but during day, it just make sense! Tighten the knob
Moving things when you opened the door
Just happened to me. I open a kitchen cabinet and suddenly saw something moving (plastic wrapping). I got really scared first but was not home alone so I did try again. On the second try I found the reason for the movement. When I opened the door suddenly, it disturbed air inside the cabinet which cause the movement of plastic wrap. If I was home alone, I probably would get really really scared.