corrosion pits happening suddenly on uncirculated large silver coins

I have been finding that some of my uncirculated world crowns develop
sudden corrosion pits and I don't know the cause or what to do to stop
further damge. These pits are definitely into the coin surface, not
just spots or toning on the surface. The coins are stored in non-PVC
safe-T flips, once they were in PVC flips but after some years I
changed the flips and cleaned any possible PVC residue with acetone. I
have seen this problem develop on perhaps 5% of my bright uncirculated
crown coins which I have owned for years. The affected coin might be
fine for years (I look at my coins closely and often), and a few weeks
later suddenly develop a pit or two. The corrosion spots are cone-like
depressions only a millimeter or so, frequently with tiny red/brown
centers, sometimes there are tiny lines radiating out. Usually only
one or two per coin, they aren't on the high points, more often in the
field randomly located. Once affected, the coin doesn't necessarily
develop more pits. They aren't associated with any green on the coin. I
can't see any associated residue on the holder that might cause it, or
on the coin. A scary problem unless I can figure out how to prevent
these for the future. The coins are bright white, have been dipped in
the past, still very bright. It only seems to happen with white
uncirculated coins, particularly crown size, not on toned coins or
copper. Anyone seen this before or know what causes it or what to do
to prevent? I have never seen this problem anywhere in the coin
literature.

My guess is that this is related to condensation somehow. That is,
moisture is condensing out of the air due to temperature drops and
focusing toning/corrosion at that point. My advice is to keep the
coins at a constant temperature (room temperature) and reduce humidity
with a dehumidifier or silica gel. Perhaps Intercept Shield holders
would also be effective, but not I think if the coin is in contact with
tiny water droplets.

Bob Leonard