It's been said several times all ready, but you were meant to find this little girl. God is using you to help children, and it's a mission to be thankful for. You may literally have saved several lives!
I can imagine so many ways this could have happened... but if she was dressed well and looked healthy and well-cared-for, I'm not ready to condemn anybody yet. I look back at my own children's escapades (including a tot who let himself out a sliding door, and closed it behind him so I didn't realize he had escaped until my neighbor called... you can bet there was a lock on that door by nightfall!)
If you find out the particulars, we'd all love to hear them!! And rather than be upset, thank God for using you in this marvelous way. You're obviously a very special person.