I have a couple of 1:25000 maps covering my area of residence. A month or so ago I dug out one of them in order to scan part of it and send the resultant image to a cyber-acquaintance. I doubt that I had referred to this publication within the last three or four years. I also forgot to put it back in stowage that evening. The following afternoon my wife came home and plonked down before me a new copy of the very same map. Having given her a puzzled look she explained that the previous evening she had looked for the 'missing' map and, not finding it, thought it best to buy a new one.