My buddy and I were in the Vane Bros. elevator after dark a few weeks back. It was our first time there. When we got near the top of the main part of the building (not the silos, higher than those), a beep - or rather, a continuous high-pitched noise - started. It wasn't coming from outside; we listened near windows to make sure. It was obviously emanating from somewhere on one of the top two floors. When we went down, it grew more faint and was imperceptible just a couple of floors down. It wasn't what I would call "loud," but you definitely didn't have to strain to hear it. The thing is, the place is completely empty, and there is obviously no activity - or anything at all - on or near the top of this place. We thought maybe we tripped some kind of alarm, but why would the alarm only be set for when someone reached the top? Moreover, why would the alarm have made a noise? Noise-making alarms, such as car alarms, are designed to be loud and, well, alarming and to drive people away. This noise was not abrasive. We thought maybe other explorers (or any other people) were there and it was some kind of equipment that they had, but we listened carefully for voices, footsteps, rustling, etc. and there was none - just silence and the noise. Since we couldn't figure it out, it was enough to spook us into making a stealthy retreat.

So my question is: Anybody have any idea what it was?