Would that be the panel (left side)........sorta slanted?
Never saw such a clean and empty basement except at a dealer.
I have had something similar a couple of times. The first time it was the backfeed valve in the city water fill had hung in the open position. When the pump was turned on when we were boondocking it back fed and shot a 15 ft. stream of water out of the hook up. The other time was when I had drained the waterheater And had opened the pressure relief valve to drain the water faster. I forgot to return the valve to the normal position. Same thing when the pump was turned on it pumped the water out the pressure relief valve. Both times I had a small ammount of pressure.