I'll chime in that an air tight box will draw moisture through the seals. Originally the ammo was put in moisture free, with dessicant, sealed and distributed full to the top. Very little air.
A used box, with older dessicant, and a large volume of air cannot handle the cold/heat cycles in a rapidly changing environment, especially charged with humidity. A cold box will draw in moist warm air and trap it. I have opened boxes with literally inches of water in them that has become trapped. I have seen the same happen to gas cans in the trunks of cars, and some vehicles are susceptible to drawing water into the distributor in spring - really hard to keep running until the cap is sprayed out and venting opened up.
Same thing for gas tanks in unused autos, or containers on a concrete garage floor - especially in spring. Cold object, warm moist air = rust.
A used box, with older dessicant, and a large volume of air cannot handle the cold/heat cycles in a rapidly changing environment, especially charged with humidity. A cold box will draw in moist warm air and trap it. I have opened boxes with literally inches of water in them that has become trapped. I have seen the same happen to gas cans in the trunks of cars, and some vehicles are susceptible to drawing water into the distributor in spring - really hard to keep running until the cap is sprayed out and venting opened up.
Same thing for gas tanks in unused autos, or containers on a concrete garage floor - especially in spring. Cold object, warm moist air = rust.