> I think that the batteries on the floor will be colder, so are prone to lose charge / fail faster (internal discharge through plates).

Cold is good, as long as the air is very dry. It will reduce self-discharge.

> Being colder (due to being on the floor) could also bring water vapor condensation on the battery surface, so that could be a path of external discharge (between the battery terminals).

Bingo. Warmer air holds more water, and as the temperature drops, because cooler air can't hold as much water, it will condensate out and form droplets. Water and moisture are bad for cells, even humidity is bad. A garage is usually not climate controlled. Some awesome folks do climate control their garage, most don't though. Whatever the %RH outside is, the garage will follow. Most US states have a surprisingly high average %RH.

whats the physical process by which water condensation discharges batteries?

I think of it this way: when it's dry, static electricity can build up, and that's why there are static shocks, when that built up electricity is released. But when it's humid, there's never any static electricity or static shocks because electricity can't build up; it is just constantly absorbed by water vapor. Water is technically an insulator, which is why distilled water is a poor conductor and salty water is a good conductor. But water vapor, even though free of contaminants, increases the conductivity of the air by reducing the air's breakdown voltage,[1] helping to ionizing the air to form plasma. Condensation helps absorb static charges by keeping surfaces moist, increasing surface conductivity. Condensation also damages batteries by oxidizing the contacts, which are steel. Water can even condense under the cathode contact, which is vented, and get into and dilute the electrolyte, leading to capacity loss and possibly shorting the cell by causing the formation of dendrites.

[1] https://en.wikipedia.org/wiki/Breakdown_voltage

Yeah I'm not sure how such low voltage can overcome the lack of any ions in the condensation in which this could occur.

There's likely contaminants on the surface of the battery which, when water is added, become conductive.

if youve ever rubbed pH paper on an auto battery youll realize there is a lot of residual sulfuric acid on the outside of the case.

That mixed with water is pretty conductive.

Yeah humidity increases the conductivity of air by reducing the breakdown voltage of air, not by conducting electricity, because it doesn't conduct electricty, because it's an insulator.

That’s what I would have thought as well.