Common cause of leaks in boot?

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What are the common things that cause a leak in the boot. The bodyshop is having the car back tomorrow to diagnose and hopefully fix my leak. So far they've looked at:

Spoiler drain hoses

And they'll be looking at the rear light seals tomorrow.

So what else causes leaks in the boot? I want to help point them in the right direction to speed things up.

 

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Anyone?! A square vent in the chassis behind the rear bumper on the passenger side was looked at at the body shop over the weekend but there is still a leak somewhere!