Has the radiator vent port broken off maybe? That would have sprayed everything on the left of the engine bay with hot coolant though so ought to be very obvious.
The only coolant thing in the area you describe is the block drain port which is a metal pipe bolted to the block that drops down to the sump so you can drain the coolant out of the lowest point of the block. That frequently gets silted up, but it should have bolt in the end. Possibly that has failed / rusted out / someone left the bolt out and it just unblocked itself...?
I would be disinclined to leave it overnight filled with plain water in this weather.
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