Strange boot leak

I found some stuff in the boot of the car wet this morning. It had been raining heavily.



I emptied the boot.



There is a damp patch in the middle of the boot floor, on the oblong "dent" in the middle. I can't see where it has come from. There are no leaks from the lights or the rubbers. I washed the car today, and it didn't get any worse.



There is no trace of wet above.



Any ideas?

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