Our 2012 Prius V had been sitting a few weeks when my wife went to drive it and the brake pedal went to the floor and it would not start. The dash lights appeared normal. It was low on brake fluid but adding some made no difference. I finally thought to put the Noco Genius 7200 on it and found just the 25% light was flashing. When it got to 50%, I gave it another try and it started and operated normally!
Brake fluid should not be low or need to be topped off on a 2012. Are you sure it was low? Any fluid anywhere? The brake primer pump runs when the driver door is opened and can pull significant power (relative to overall 12v battery usage on a Prius). How long is a "few weeks"? Was your car brand new off the boat, or did a dealer let it sit and most likely die on the lot a few times?
Funny that the pedal went all the way down. I would be interested to try the following experiment: - disconnect the 12v - let car sit for a few hours - try brake pedal height. My understanding of brakes on Toyota hybrid is this. During normal driving the pedal is only sending pressure to a stroke simulator, a device that gives the pedal normal back pressure. The pressure for hydraulic braking is created in the electric actuator. When the car is off or electric power has failed a direct link from the pedal to front brakes is present because of the position of a few normally closed and normally open hydraulic valves that are not powered.
