I would suggest testing the drains once more. Do it one drain at a time with a metered amount of water. Now that you know where the drains dump, you should be able to recover the water and compare the recovered amount to what you sent down the drain. It sounds like one or more of the drains is diverting to somewhere, where the water can collect, and the motion of driving released the water from this collection point to the floorboard.
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