I'm sure he's happy about that.
I know that my ex got out once because there was no room in the jail. Of course he was supposed to go back for court but never did...
You may want to check your county jail registry (online) if you are interested in why he was released. It seems that in most counties you can access that info online now. It may give you info on whether he bonded out or was released for some other reason.
Just keep doing what you are doing to take care of yourself and your kids. We can never know what tomorrow will bring. Maybe this will be the turning point in his life (my sarcastic self is saying... yeah right. But that's only because of my experiences...)
(((hugs)))