House -sitting safety?

Is it possible the footprints were left by a mailman or delivery man? Between those hours I can see how it's probably unlikely. If it's going to drive you nuts -- which it definitely would me, call the police. Explain you're probably over-reacting but ask if they can have an extra patrol car drive by the house a couple times during the night. As long as you're in an area were the crime is relatively mild they'll be happy to do it.