Bit of info I found on off gassing
"Dissolved gases are another part of the taste. As water sits out, small amounts of carbon dioxide dissolve into the water. This forms carbonic acid, which may lower the pH just slightly. Tiny amounts of other gases, like acetone and aldehydes, may dissolve in, too. “It’s very difficult to have a pure water sample for long,” says Richardson, who would know because she has to use completely pure water in her research. Leave purified water out for even 30 minutes, and it quickly becomes impure. “It’ll drive me nuts in my lab,” she says. Temperature plays another indirect role, too: Cold water generally holds more dissolved gases. Additionally, some oxygen may dissolve out of the water over time.
But if you start leaving your water out for days and days, then, finally, microbes enter the equation. Algae make earthy-smelling molecules like geosmin and 2-methylisoborneol, to which the human nose is incredibly sensitive. Geosmin, incidentally, also gives rainstorms their post-rain aroma and catfish their muddy taste."