That dog behaviour is what gets me every time. Animals don't lie and they don't "misinterpret" sounds the way we do, so when a dog flat out refuses to go outside until daylight that tells you something very real was out there.
What did the walking pattern sound like? Was it consistent, like a regular gait, or more random and stopping and starting? I ask because I had something similar years ago up in the hills outside Portland and what struck me was how deliberate the movement sounded. Not like a branch scraping or debris shifting around. Actual footsteps with weight behind them.
Did you check for prints or any physical evidence in the morning? And how big is your dog because a small dog spooked is one thing but if you've got a large breed that went full refusal mode that really changes the picture for me. Curious what others make of this one.