What is causing water ingress to the boot of my Ford Focus?

I regularly find half an inch of clean water in the boot well of my Ford Focus. Exhaustive tests with a hose pipe applied in sections from the bottom of the car back end don't result in any water under the spare wheel, nor does prolonged activation of the rear window washer. I can only think the water comes from the road, but how?

Asked on 16 February 2013 by AB, via email

Answered by Honest John
Via the cabin vents situated behind the sides of the rear bumpers. They are supposed to vent through one-way flaps, but if the flaps fail then water splashed up against them is admitted to the car.
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