I drive a 2010 Prius V, 8500 miles. We took a first long (non-commute) trip in the new Prius and, of course, encountered the first problem with this car 500 miles from home. To deliver the punch line first, my problem was at least on surface solved with replacing the 12-volt battery. I need to know how this happened, though. I'll recount the circumstances. After 500 mile drive, parked car. 65 degree ambient temp. Two hours later, went out to remove some items from rear hatch, and discovered that the all exterior and interior conveneince lights were out but the display panel on dash was on -- as if I had not shut down the system when I left the car earlier in the evening (I've never failed to do this before). Car started fine, and I moved it to a more convenient parking spot (very short drive). Two days later, 65-75 degree temperature, car started great, and drove the Prius 40 miles, parked, and back 40 mies to a parking garage while we ate dinner. After my earlier experience with the display being on, I was explicitly careful to turn off the car with the power button, then turn off the headlights, then lock. When we returned to the car 2 hours later, the headlights, fog-lights were all on, car was locked! Strains of Twilight Zone theme music ... Car started perfectly, drove 2 miles, then I parked: made certain car was turned "off" and lights were out. When I came out an hour later, park and fog lights were on, faintly as I recall (though no headlights) and the car would not start up. Clicking noise upon hitting start button, lights flickered at frequency of the "click," then all was dead: no interior lights, no ability to lock electronically, or to open back hatch from outside. Briefly I did read the infamous "Parking brake" message, directing me to put on, then off, the parking brake and park on a flat surface -- a tall order with car dead!) Rather inconvenient! Towed (dragging car onto a flat-bed truck, as front wheels were locked and could not be released)to Toyota dealer. Toyota dealer replaced the 12-volt battery, did comprehensive tests, found no other problems. We drove the 500 miles home yesterday, and the car seemed to worked perfectly. But why and how did this happen? How would a failing 12-volt battery cause headlights to go ON? Why, even if I did (and I don't think i did) leave lights on, wouldn't the lights just go out after a minute or so delay? Is there something different I should do -- trickle charger, install different (Optima?) battery, get checked for early-Alzheimers onset? I make no claims for being an auto-techie, but would practically like to know how this occurred and assure it never happens again!