Pull the cover of the drip tray off & watch the rear LHS of the drip tray ( about an inch in from the very LH corner) when you switch off the coffee button. If all is well the pressure relief will come out into the drip tray there.
In any case with the Silvia you dont normally hear the same big gush of pressure relief that you do with a semi-commercial or commercial machine. Also if the pressure relief on your Silvia were not working, youd get a wrist full of hot burning coffee dregs blowing out of the group handle/filter when you go to release the handle.
Water on the top of a correctly dosed puk can only mean one thing....coffee too fine...except that in your case you have mentioned you are only getting a +slight+ *imprint from the grub screw. That grub screw sticks down an awful long way, so if you are not also getting an imprint from the edgeing ring of the shower, then you are still basically underfilling your filter (even if only by a small amount...which would be the reason why the puk is firm enough)...in which case the wetness is probably caused by under dosing OR a combination of under dosing as well as coffee too fine.
Hope this helps.
Your supplier should be able to tell you this!