There are thousands of these things, and in the past even more. There are loads of cases of older people having had them around for years unaware that they might have posed any danger. So it's not really unusual. Wikipedia sites between 2006-9 over 15,000 items of ordnance were found on construction sites in the UK. All of them will have been lying around somewhere at some point, and many known of for a time, and I doubt that this one's the only one that was ever employed as a doorstop!