I have read that smokeless powder comes in plastic bottles is for safety reasons. The plastic will melt, preventing the powder from being contained in an enclosed container, and will just burn, probably causing other plastic bottles to melt, with more powder burning but not causing any explosions. Primers would probably go bang as well from the heat. I've also read that loaded ammo will pop off like/similar to firecrackers as the loaded round is not in a chamber, so any pressure from heat will not equal to what occurs when a round is fired in a chamber.
I store reloading components on shelves, in cabinets, plastic storage tubs, etc., none of which are airtight in a climate controlled environment. I store loaded ammo in plastic ammo boxes and crates, none in airtight containers.