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I also suffered some years ago from undetectable leakage. Then I used a small endoscope to look inside the beam in the bonnet, where the horn is mounted. I found that internally the water was leaking to the interior due to a cracked seal.
To avoid going on a hunt to repair all seals, I treated the bonnet inside with anti corrosion grease, also sprayed this inside the beams.
This stopped the leaking, and also prevents any rust.
The other source of leaking was due to cracked plastic covers on the inside of the doors, so I replaced those covers with home made new ones from PVC, that stopped the leaking from that side.
Now my car is 90% leak free, but in heavy rains, still sometimes some water gets in at an unknown point, So I frequently check the underside of the carpet to see if it is damp, but usually this is fine, and when needed I lift the carpet to let it dry for a few days. This is in my case the maximum achievable result.

Tnjk. I don't see exactely what you mean by the bonnet beam but i think i don't have that problem ( knock wood).

I did the renewal of the door platics, resealed the rhd steering axe blind. Refitted and sealed the plate with the clutch master cilinder (small leakage there). Sealed the unused grommet next to the wiring grommet etcetera.
Thats why i was bit frustrated but determined to find that last bl...dy leakage.
Hope it is the last indeed (as for now and some years if i'm lucky).

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