photos via here and here
This beautiful Art Deco-style skyscraper downtown was my old office building.  Built in 1929, the lobby downstairs retained a lot of its classical Art Deco design and decor and a great number of tour groups would pass through and marvel at the ornate ceilings and stained glass windows.

Our office was on the 8th floor of the building and was shaped like an "L."  All of the analysts sat crowded together in a bull pen and about 20 feet from our desks was a closet where server equipment was stored.

The junior staff on our floor would often still be at work long after every one else in the building had left for the day.  During one of our many late nights, I was at the office with a coworker - let's call her Paris in case she becomes famous and sues me for sharing this story - and we were getting deliriously tired and hysterically giggly.  In the dead silent of the night, besides the click-clacking of our fingers on the keyboards, we suddenly heard faint music and murmuring coming from the direction of the closet.  Except it couldn't be because that closet was locked, empty except for electrical equipment and only accessible by building maintenance.  All giggling stopped immediately, our ears strained to catch the music again, but it had faded.

We both reasoned that it was getting late, we were both incredibly tired and perhaps both imagining things (the same things...).  At around 1 o'clock am, I went home to take a shower, which would wake me up, and planned on coming back in.  For the 30 minutes that I was gone from the office, I had received a barrage of text messages.  The text messages were from Paris and read:

"I'm hearing noises again and think there's someone else here."

"You better hurry up and come back!"

She was fine when I got back to the office, if not a little spooked, but had heard more sounds while I was gone.  I shrugged it off as an overactive imagination coupled with the 'music' we heard earlier.  And then, it seemed like only a few moments later, there was a banging, like that of a fist pounding, on the wall right by my desk.  We both stared at each other, uncomprehendingly.  The only thing that was on the other side of the wall was an elevator shaft.

That night we finished our work as quickly and LOUDLY as we possibly could and scrambled out of the building.  Later that week, we casually mentioned the noises that we heard to the security guards that manned the lobby.  Instead of dismissing us as silly, scared girls, they mentioned that they particularly disliked working the night shift.  People reported hearing noises from closed off rooms and seeing figures where people were not supposed to be.  And when the security guards would approach these figures, they would vanish.