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I don't know exactly what chemical (besides water) is in Dexcool, but chemical compounds other than water actually do evaporate when exposed to air. Take alcohol (like 90% rubbing alcohol) for example. It not only evaporates, but it does so faster than water does.
At one point radiator coolant contained glycol (among other things), which is essentially an alcohol. So it's also possible that other components of Dexcool evaporate just as fast as the water does.
I wouldn't sweat the details here. Just refill with Dexcool if you are low. The amount (of what ever chemicals, including water) you are losing to evaporation is still small compared to the overall coolant volume in the system.
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