I have loads of creepy stories, but this is one of my favourites...a bit long but good...
In 2007 I finished college and was offered a posting as an admin in a small town working with local law enforcement. It was a 6-month contract and paid really well so all within a week I accepted the job, found a furnished place to live and took off for my adventure.
The town had just over 15,000 people and the apartment I chose was on the top floor of a three-storey building that was built in the 1960s. It was quite large and had nice hardwood floors. The walls were somewhat thin and I heard my neighbour a few times, especially at night when it was quiet. When I first got there I knocked a few times with the intention to introduce myself but they never answered even though I thought I had heard them inside. After a week I assumed maybe it was a reclusive person so I slipped a note under the door saying hello.
I worked with the same officers all the time on rotating shifts and three weeks into my stay I was into the afternoon shifts which meant I was getting home around 2am. As I came down the hallway of the building I saw a light coming from under my neighbour's door and saw the shadow of feet under the door and the peephole went dark as if they heard me and were looking to see who was in the hall. I figured if they wanted to say hello they would have opened the door so I simply waved hello and went into my apartment. Over the next couple of weeks this happened 3-4 times again. I was pretty sure I had a reclusive but nosy neighbour.
Then one night after work I crawled into bed at 3am and could hear that my neighbour was moving around in the room of theirs that adjoined my bedroom. About ten minutes later the noises got louder and it started to get me worried. They had never been this loud and it almost sounded as if they were being attacked or having a heart attack and knocking things over. I ran out of the apartment and started knocking loudly on their door but they didn't answer. I called out to them saying I was worried about them and if they didn't answer I would call for help. Still no answer. I ran back into my apartment and could hear bangs through the wall so I called the police station and asked my co-worker to send an officer over. I went and knocked on my neighbour's door one more time but no answer and then headed down to the lobby to wait for the officer who showed up within 5 minutes. I had gotten to know him a bit and we went up and knocked on the door but still no answer. I took him into my apartment and we could still hear loud thumps every 5-10 seconds and then a huge bang. The officer asked if I could go get the building manager to open the door. She lived one block away so I said I'd run over and get her. Luckily she was a night owl and watching TV. I told her that my neighbour was being really loud and wouldn't answer the door so she grabbed the keys and we headed over. The officer was waiting for us in the lobby and she asked which apartment it was and I told her 304. She looked at me weirdly and said 304 was empty. I said I had heard my "neighbour" making noise nearly every day and had seen the light on and shadows under the door a few times. The officer said he heard the noises too and asked if 304 was the apartment that was above the carport and she said yes. He replied that someone could easily climb up onto the carport and perhaps there were squatters in the empty apartment. We went up and the manager unlocked the door and the officer opened it. The apartment was dark and he turned the light on. He went in and the manager and I were peaking in through the doorway. He went into the bedroom and came back out and said it was empty. We went in and the place was spotless. All but two of the windows were locked and no sign of squatters. We all just stood there scratching our heads. The officer looked at me and said "You're not crazy, I heard the banging too". I said maybe we scared them off and they had come and gone through the unlocked windows. We double-checked that all windows were locked and headed out. On our way out I looked down, and there on the floor was the note I had slipped under the door a few weeks earlier. We locked the apartment and I said I'd keep an eye out and call them both if I ever again saw the light on or heard noises.
Fast forward almost six weeks and I was again on afternoon shifts. I came home at 2am and there was a light on. I quickly went into my bedroom and could hear thumping from next door. I called the station and asked for an officer to come out and then called my building manager. I slipped down to the lobby and waited for them. A different officer showed up and I knew him too. My building manager arrived and all three of us headed upstairs. The light was off and the building manager unlocked the door. The officer went in and everything checked out. We checked the windows again and they were still all locked. I apologized and tried to rationalize it that I had just come off of four days of 14-hour shifts and was tired, but I knew I had seen the light on and heard thumping.
Two weeks later the apartment was rented out and I got a chance to greet my new neighbours. I told them to keep an eye out because we were sure squatters had been coming in and out and I relayed the stories to them.
My final week there, the station had a BBQ and we were talking about the squatters that no one ever saw. I told the first officer about the second call out and how all the windows were locked that time but I knew I had seen the light on and heard thumping next door. He asked if I wanted to hear something "funny"? and I said sure. He said he was going to tell me earlier but didn't want to freak me out. He proceeded to tell me that three months before I moved in, he personally responded to a call to my neighbour's apartment for a very bloody suicide. The person had done the deed but had second-thoughts and called for help. When they arrived the person was unconcious but had obviously panicked and a chair, lamp and side table had been knocked over and there was blood everywhere. Unfortunately, they passed away before getting to the hospital. I just stared at him and he reminded me he had heard the banging too the night I called him over.
I asked the building manager about it the day I moved out and asked why she hadn't told me. She laughed and said "Oh honey, I believe in ghosts but didn't know if you did too and I didn't want to scare you. I know you heard and saw things. I believe you and I doubt it was squatters".
EDIT: Davidandsarah08 said I should x-post to /r/TheTruthIsHere so I have. They allow for reasonable explanations/debunking and I'll post there when I get home from work tonight as to how I rationalized with myself as to what happened in order to keep calm and sane! http://redd.it/1fbxc7