I worked at a large university in facilities and had a master key. I would use the tunnel system that was under the older part of campus to get between buildings when the weather was unappealing. It was an arched brickwork steam tunnel built by prisoners in the early 19th century. The prisoners manufactured the bricks themselves and laid the tunnel. It was filled with their graffiti etched into the bricks which was super interesting but very spooky.

One day I was using the tunnel and was walking through a section that had access to the basements of old buildings, that because of sinkage and subsequent renovations decades ago, no longer we connected to the building above. The basements were off the main tunnel and were only used for equipment storage during projects but otherwise were old classrooms with blackboards and some old desks. So, in summary, very creepy, old, dusty, and dark, subterranean, abandoned classrooms.
As I was walking past, I definitely heard whispers from one of the classrooms. I am not superstitious at all, but I heard what I heard. So I walked into the room and I didn't hear it again. Just rows of old dusty desks. I hear it again though as I walk out and then I pinpoint it is coming from under one of the back desks.

I check it out and there is a radio fell off someone's belt turned way down. One of the other facilities guys was taking a nap, turned down their radio, and slouched down in the seat far enough that the radio clipped off his belt. Nearly had a heart attack.

Ended up just returning the radio but made sure to use the abandoned radio trick a few times to scare my coworkers.