sudden battery failures with no warning signs have happened to me on many an occasion. As in, car started fine, I got to my destination and parked, returned to the car 2 hours later, and the (3.5 yr old) battery was hardly able to turn the engine over. New battery, everything is normal again. The last time it happened was I left after work, car started fine, went to a gas station, turned off the ignition, filled up with gas, cranked the engine, and all I got was a starter click. Fortunately there was a store selling batteries just across the street, got the replacement installed, and everything is back to normal again. No, it wasn't corroded cables or connector nor alternator; replacement batteries lasted 3-4 years.