FULL DISCLAIMER: While all of the spirit encounters I've had have been almost certainly paranormal and cannot be explained away, this is the only one that MIGHT have been partially created from my imagination--but I don't really believe that's the case, for reasons of which I'm about to explain.
A couple months after I started my last job at a gas station, I was left alone to close up the kitchen. After my manager left and I was about to start washing some dishes, I heard a child--perhaps a little girl?--laughing in the back room (which was right next to the kitchen). The store manager sometimes allowed her granddaughter to come in while she waited for said manager to get off, and I thought that's who it was, but when I went to look there was no one there. I chalked it up to imagination and put it in the back of my mind.
A few weeks later the assistant manager and I were talking and--according to him--there was a time where he and a friend of his were talking at work. He was working the night shift, and the two of them were the only ones in the building. Things had slowed down so they were cracking up. One of them made some joke and once they both stopped laughing they heard another, much deeper chuckle from the back area of the building. Both of them were creeped out and the manager went to investigate to see if anyone was there, checked the security footage, and so on. While he said that the laughter hadn't been picked up by the cameras, both he and his friend heard it clearly (and of course, this is assuming he was being honest).
A few more weeks pass, and my boyfriend had started working at the gas station as well. Without having ever been told about my story or the manager's, he came up to me a few hours before leaving and told me that he had heard a voice of an elderly woman chuckle and say, "Mr. Paul." When he turned around, no one was there. Keep in mind that he heard that crystal clear, EVEN THOUGH at the time one of his ears was blocked, and he was having trouble hearing me if I stood right beside him.
At that point I was really curious about what was going on and decided to bring my pendulum to work. I got there thirty minutes before I was scheduled to clock in and hid where I knew no one was likely to find me. I grounded myself and did the usual things one needs to do when they're using pendulums. If you're not familiar with them, I highly suggest you look into them. They're very interesting, very useful, and I can say I've yet to find that something my pendulum guided me to was wrong. When I asked if there was a spirit that was staying in or around the building, it swung "yes." When I asked if said spirit was the same that I and others had been hearing, it swung for yes again.
The next day or so I heard voices talking in the office (which was above the kitchen), but it struck me as strange because no one was supposed to be up there. When I went to investigate the noise had stopped and, of course, no one was there, but when I got back to the kitchen it sounded like there were people talking in the front counter area. I looked there, but there was only one person working for the front counter, and she said she hadn't had a customer in the past twenty or thirty minutes.
I've since quit the job, but I've been thinking about going back to the area to investigate. If I do, and if anything significant and / or noteworthy happens, I'll be sure to update this.