The sun stole your chlorine. Back when I was a kid, I would occasionally buy goldfish. When ever I set up a new bowl, or cleaned the old bowl I would have to let the water sit for a day so the chlorine could escape from the water.
Chlorine is happiest in normal temperatures as a gas. To keep it in a liquid state we have to do funny things to it. If you simply put chlorine in a pool full of un-stabilized water it will happily find ways to escape into the atmosphere.

