I am going to have a guess and suggest the moisture seeped out from the point where you cut the squash off the plant. When squashes ripen this connection gets dry but when unripe a lot of sap can exude from the cut. You would need to damage the skin quite badly for it to emerge from the body of the squash.
Just a guess tho. On second thoughts, maybe condensation if the squash was colder than your kitchen?