On my way back from lunch today, I was walking down the stairs when I heard a loud "thwack" and turned round to see that my security pass had come loose from its clip and fallen to the floor.

The stairwell is quiet, and sounds reverberate off the shiny, hard concrete surfaces, which has the effect of amplifying them.

If I had dropped my pass in the street outside the building, I might not have heard it hit the ground. And I might have gone blissfully on my way,...