Modern cars are constructed by spot-welding the panels together. This is strong enough to hold your car together, but it doesn't make them waterproof, as the seams aren't welded all the way along.
Instead, they put a sealant along all the seams. This is flexible when it's new, but over time it can dry out and crack, which lets through water.
We see this quite often around the tops of the rear gate on hatchbacks, especially around the hinges, but on this Vauxhall Astra it's happened around the front scuttle. It is a simple matter to just re-seal it with a good quality automotive sealant.
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