Thanks to all for the suggestions and insight. Alas, I don't seem to
be much closer to a solution.

The ice doesn't appear to be coming from the water dispenser in the
door, because the door hinges (where the water tube is, I believe) are
outside of the freezer bottom. Any leak there would run out onto the
floor.

The ice also doesn't seem to be coming from the ice maker refill
operation. I turned off the ice maker for several days, and eventually
ice still formed on the freezer floor.

I will try probing the condensate drain with a more flexible wire, and
also heating it with a hair dryer to thaw any frozen blockage.
However, the area around the drain is clear of ice, but the ends of
the drain "tray" are filled with ice. It looks like the tray filled
with water which froze before it all drained out.

Someone suggested hanging a copper wire from the defroster heater unit
into the drain. Is this heater behind the plastic shield directly
above the drain, on the rear wall of the freezer?

It's interesting that the ice appears only after several days with no
ice. It doesn't gradually appear over time; one day there's a sheet of
ice.

I'll keep looking; thanks again.

Mike Dodd