If you have strange things going on in your house, first determine if there is a human prankster involved. After that is ruled out, set up hidden cameras to find out if it is due to an older home, thieves, or pets.
While some schools of thought hold that unusual events can be ascribed to an external agent, others think that the person experiencing the event has an unconscious effect on it.
That depends entirely on your beliefs. There could be a spirit. Personally, I believe that each house has an 'imprint' that every person who has entered has left.