If the car hasn't had a knock or a replacement scree chances are the plenum (space under the plastic trim below screen) is blocked with leaves and crud. Simply remove the plastic trim and clean out. The trim is sometimes awkward to remove because screws have seized - be careful.
If it has a sunroof, the drains get blocked.
If the car has had a replacement screen it could be leaking.
If the car has had a knock (like mine) it could have been repaired without time spent resealing seams etc.
"A" pillar door seals can leak over time, especially if they have been removed and replaced a couple of times.
The membranes behind the door cards leak if they have been tampered with, especially round the speaker hole.
There are grommets behind the arch liners that can leak.
Worst case is the heater matrix leaking - usually older/high mileage cars. The dash has to come out to fix this.
Do a search on this forum or just google 'alfa water leak' and you will find lots of Alfa's have leaked and some continue to