Astra J water leaking into boot

Im working on a friends Astra J hatchback trying to find where water is getting in to the boot and pooling in the spare wheel well.

So far I've poured loads of water over the rear doors, rear windows, boot door, petrol cap well, and light clusters. Nothing has let water in. Ive had the rear light clusters off too. I've removed the plastic boot panel where the striker plate sits. No sign of water ingress there.

I'm thinking maybe it only leaks when its moving, letting spray in somewhere.

Any ideas where I should look? Thanks