A chair just moved upstairs above my head

I am in the basement right now, working (I work from home). Holly is next door, visiting our neighbor and drinking coffee. The house is very quiet. The nosiest thing is my tapping on the keyboard.

About ten minutes ago I heard a chair or something similar-sized move across the floor above me. It slid only a few inches then stopped. The dining room and the kitchen is above my head as I sit here in the basement. Both rooms have chairs. The kitchen has an island with some bar stools around it.

Jack, my biggest and oldest dog, was upstairs at the time. I heard him walk over to where the sound came from, and after a few seconds, I heard him walking awkwardly down the wooden steps to come and jon me.

When he came down, he stopped, and stared at a spot over at the far side of the basement. He stayed there for quite a while, then went over to that area to investigate. He wasn't staring at the floor, but at a spot about the same height as his head.

Goose bump city! I almost considered that it was a good time to take a break from work and go work out at the gym...