Heavy walking up new basement stairs

It was late last night by the time my wife and I got home. About 9 pm I took the dogs out and when I reentered the house I was tired and simply sat down at the kitchen table.My wife had already retired to the library to catch up on her E mails. One of the dogs got up in another of the kitchen chairs, turned around several times and laid down. The other dog had left the room and I assumed she was headed for the library. I hadn't sat there long before I head the unmistakeable tread of heavy boots or shoes coming up the basement stairs. I was at the steel door leading to the basement fast. I swung open the heavy door and observed no one on the stairs. I turned on the basement light (switch at the head of the stairs) and could not see anyone or anything alive. The basement stairs were just replaced this summer and showed no footprints of any kind. As anyone who happened to be in the basement would have had to track through mud to get to the stairs, muddy prints should have been evident. The mainroom of the basement under my house is wet most of the time due to a spring that used to run through it but was never totally diverted. This is a very old house(1857) and the spring, at that time, was used to keep dairy products cool. When I bought this house I wondered  why there was a steel door equipted with a deadbolt on the cellar stairs. Friends of ours said it was required by the state when the house served as a rest home for a number of years. That could be correct, but why the deadbolt? I hadn't been using the deadbolt lately; but I am now.