If you have an enclosed porch the wellies may have been taken by someone delivering leaflets. A few years ago I happened to be in my hall when I heard the sound of something dropping through the letterbox in the porch, then the sound of the porch door handle being turned. The porch door was locked so I wasn't worried. When I looked out I saw a leaflet deliverer walking away from the house. Funnily enough I think there were a pair of wellies in the porch at the time.

I sometimes think that being a leaflet deliverer must be a good way of legitimately going from house to house, while also sizing them up for potential thefts or burglaries.