Looks like the rubber boot that sits behind the lateral lock cylinder may be leaking. The rubber boot is n 964 301 051 01 shown in illustration 302-10 in PET.
Also you could check there is a small socket head bolt that belongs in the lowest hole on the bottom right of the join in the picture, small one like a M6. If that's missing it might leak I guess.
Attached image shows you what that rubber boot looks like with the slave cylinder removed
