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.

--
Andrew Gabriel