Interesting, I worked on a project a few years ago on a large electricity substation in Denver Colorado, the oldest one in the city. Upon lifting the covers on some outdoor ducts we found heaps of shoes and boots. Always just a left or right, never a pair together. I asked the young subtech at the time what was going on as it looked frankly quite strange! He didn't have an answer but the following day we had an older guy on site who laughed and explained that it was foxes. They were stealing from peoples doorsteps and yards and bringing the boots back and hiding them around the substation which is where they lived. Some weeks later I got a great photo, I'll try and find it and post it here.