Fast food ghost

I worked for the fast food industry for about 10 years give or take. I always chose to work mornings so I could get my work done then go home or spend time with my girlfriend. Working in the morning I never thought of anything scary or haunted, it was usually the customers that scared you most.
One day a co-worker which was my manager and I were hanging out at the front counter. The place was dead which was pretty normal about this time of day. We always died down after lunch and before school was out. Anyways her and I were talking and joking about work. I grabbed a towel and went out to wipe tables. It was not busy so might as well work to pass time. She was behind the counter and we heard a loud slam from the back of the store. We both froze and looked at each other. We were the only ones in the store.
So her and I went to the back just to see if maybe the back door was open and maybe someone was messing with us. No one was there I checked everywhere. The back door was locked and the alarm would have went off if someone had opened it. After we established no one was in the store the fear kinda set in for a second. If we were the only ones there then what was the slam? So we started closing doors to the equipment just to try and recreate the sound. I went to the microwave and opened the door and shut it I didn’t just barely do it either I really slammed it closed and both of us started freaking out. It was the exact sound we heard. I know loads of people would say well maybe wind closed it or a broom fell. Problem is we never kept the microwave door open there wasn’t enough room if it was. So something had to open it to close it.
Now I know that isn’t the most compelling or scary thing ever but at the moment knowing nothing could have a done it that could be logically explained made me start wondering I asked my general manager if she had ever heard or seen anything. She had a few little strange occurances but nothing that screamed haunted.
I no longer work for that place and from time to time think about that moment and wish I would have tried to make contact.