but as long as you keep it covered and away from possible contaminants and remember to feed the bugs you want, that's not really likely. It's best to keep it in the fridge once it starts working, though. You'll know a starter is past its prime or downright contaminated because it will no longer smell fresh and sour, it'll smell distinctly funky. You won't like bread baked with a bad starter.
Back in the psychotic 60s, people who went the 100% back to nature route used to mix up flour, water and a little salt and let it sit out at room temperature until it got inoculated by wild yeast in the air and doubled. Sometimes those wild yeasts turned out a decent loaf of bread. Most of the time they didn't, but it was always edible. Well, almost always.
I'm beginning to think bread baking is the least fussy thing we can possibly do in the kitchen. The one thing to remember is never proof that yeast, hot water has killed off more yeast than anything else ever has.
As for proofing the first mixture overnight, that's what most of us are doing these days. I've done it for 20 years, ever since I realized that same day bread at 6000 feet tastes like cardboard. It takes overnight to develop any flavor, at all.
However, if you want to get a breadmaker bible that will explain it all, try "Crust and Crumb" by Peter Reinhart.
http://www.amazon.com/Crust-Crumb-Master-Formulas-Serious/dp/1580080030