Knock Knock Knock

It was about 9pm, just last week, and I had settled my 2 1/2 year old son into bed about an hour previous. I worked all day so I decided to go to bed early. I went upstairs to the bathroom in the 75 plus year old duplex I rent to participate in my nightly ritual. I went into the bathroom and closed the door tightly. I emphasize tightly because anyone who has experience or knowledge of old houses knows that the wood expands and contracts more and if I didn't close it tightly it would swing open. I took out my contact lenses and put on my glasses and was brushing my teeth when I heard very distinct, loud knocks on my bathroom door. Three knocks, slowly, in a row. I froze in fear!!! This has never happened before. I didn't know what to do. I knew I locked all the doors before retiring upstairs. My son could not possibly get out of his crib and open his door and knock without me knowing it. I didn't move for a minute. I just waited. After about two minutes the door made a loud 'bang' and slowly swung open. I jumped back and waited. I didn't see anyone or anything. I didn't hear anything. My sons bedroom door was still closed and there wasn't anyone else in the house. I stood in fear waiting and staring out into the hall, hoping to see some explanation for this but saw nothing. This house is old and I don't have the complete history on it but I think someone wanted to use the bathroom.