12V Battery Already Dead on a 2.5 year old Prius?

Guess my definition of depleting and yours are different. We'll move along.
Still to the point I was making, however, the 12v battery being "drained" will cause the hybrid to attempt to charge it at start up. If the car isn't run long enough to recharge the hybrid battery, it could do weird things to that battery. Toyota has many safeguards in place when it gets below 2 bars to try to save the battery. A jump may not be enough to start the hybrid system at that point.

 

If the OP is still with us, the battery had at least a 1 year warranty and should be replaced by the local dealer. OP should also put a trickle charger on the new battery from time to time.