On more careful inspection, I'm convinced my initial theory is bogus. If anything, water coming in this way would end up in the boot. Even if the drips were to run along the rim bottom all the way forward before falling off, they would be on the outside side of the door gasket, so now I don't think this entry mode is feasible at all for the passenger compartment.
About all I can think of at this point is water is coming in through the somewhat typical failed door membrane or similar door leakage modes. It could be possible for the water to run under the carpet and collect somewhere such that the original ingress path is not evident. The only remaining evidence is the collected water. How it got there has dried up so there is no possibility of back tracing to the point of entry.