I was servicing my Keston Celcius 25 yesterday.
Over the years, I have noticed slight rust staining on the bottom of the inside of the cover along the metalwork seam. Obviously some moisture is ending up there, but I never found the source. The sealed system has lost no pressure in 3 years, and when running a garden hose into the end of the flue, there is no condensate leak. I leave the automatic bleed valve closed, so it's not that. The amount of water must be small as I have never seen any there, and the staining does mostly just wipe off, and there's no sign on the outside of the case/cover.
Anyway, I think I found the source. I was running the boiler with the cover off. The boiler is hard up against a wall on the right hand side, which is bare unpainted plaster. The air intake pipe faces this wall. When the run-on timer finishes and the fan stops, moisture laiden air convects out of the air intake. I noticed this because without the cover on, enough was coming out to make the plaster wet and change to a dark colour. With the cover on, this is going to condense on the inside of the cover and run down to the bottom.
To prevent any long term problem with this eventually corroding through the case, I have run a bead of silicone seal around the inside corners of the cover bottom where there is a metal seam. The metal surface seems to be sufficiently protected by the painted enamel finish. The moisture probably evaporates next time the boiler runs for a while. This might be a useful tip for other Keston owners here who service their own boilers.