That sounds properly unsettling mate. Dogs reacting is always the bit that gets me, they pick up on things before we do and you cant really explain that away.
We had something similar round at my sisters place outside Glasgow a few years back, heavy footsteps on the flat roof section of her kitchen, dead slow and deliberate. Her dog was going absolutely mental, scratching at the door trying to get out. Checked the roof the next morning and nothing, no marks, no sign anything had been up there.
Did yours sound like it was moving in a pattern or just random? And was it one set of footsteps or more? Would be good to get more detail from you on this one.