Lost Frost plugs

We had installed the rad and hoses but left the drain plug on bottom the the rad was open.No water or antifreeze had been added..I guess we should have in retrospect..

I cannot think of any reason water would be in there unless it was tested after the rebuild ??

If there was water, why didn't the ice go up instead of forcing the plugs out..

I realize that it is now a moot point but I like to understand the whys....