Your point is not really correct Gollum. Whether an estate or hatch back or saloon, water will travel around the rear tailgate / around the boot aperture, and if there any areas with poor seals etc en route will be a source of water ingress. My grandaughters 06 Ford Fiesta got a puddle in the spare wheel well everytime it rained. I traced that to a seal behind the rear light cluster where a plastic bracket housed the wiring loom and had a sponge like seal (? ::?) against the body. Removed the seal, and replaced the bracket with a bead of permanently flexible DumDum, and its been dry as a bone ever since.
To be honest, unless something very obvious, somebody will have to get in boot whilst another hoses the car over.
Although it may he hard to believe, but from work related experiences, a hose on a car to try and find a leak doesn't always replicate actual rain water falling from the sky! Issues with cabriolets for example that were often prone to water ingress sometimes could only get replicated parked in the rain instead of using a hose on a tripod!!
Assuming there are no corrosion issues that may be allowing water ingress, I found the leak into my boot appeared to coming through the spot welded seam in the front edge of the boot lid aperture, just behind the rear window on either side. You could see daylight through the seam if you shone a torch behind it. I applied a thin smear of Dum Dum both sides of the seam, forcing it directly into the seam, and its has been fine since. And it is barely visible unllike brushable seam sealer.