OK, mine's not as scary as some I've read on here, but it still freaked me out at the time! It's also a bit long - Sorry!
I work in a clothing store, which around 10 yrs ago had been a Woolworths. Lots of members of staff hated using the basement stock room, as they claimed it was creepy and something was down there, watching them, and some had claimed they'd seen a figure of a woman. I personally had never experienced this, but the basement did have a sense of unease about it, which I chalked up to the fact that it was below ground level and the back storage areas had a 'cave like' appearance and a claustrophobic catacomb sort of layout. All the staff jokingly referred to the ghost in the basement as Mary/Scary Mary - the name had been passed on by one of the women who'd worked in the store when it had originally been Woolworths, as that's what the Woolies staff had referred to the ghost as.
Anyway, 2 years ago the store moved premises to a much bigger, brand new purpose built store, just in time for the Xmas period. However, the lease for the old building didn't run out until several months later, so whichever manager was on duty also had the keys to the old building, in case the alarm went off, as they had to maintain it until they handed the keys back.
One night the staff were working night shift over Xmas and the alarm went off in the old premises. The on duty manager and another member of staff who had his car with him drove to the building to reset the alarm and check it hadn't been broken into. They got there and had to check the building floor by floor, starting with the attic stock room, offices, 2 sales floors and then finally the basement. When they got to the basement they could distinctly hear the sound of clothing rails being pulled back and forth on the sales floor above them and footsteps of people walking around above their heads. This completely freaked them out, as there was nobody in the building and all the clothing rails had been moved to the new store, so they quickly locked up, set the alarm and got out.
They drove back to the new store and all the staff were in the new stock room, completely immersed in this creepy story in the middle of the night, as they recounted what happened. As they finished the story one of the women said 'It must of been Mary, she must be lonely' and just as she said it, a full box of equipment fell to the floor with a huge bang. Everyone was completely terrified at the time!