Joe,

As was mentioned, it is the small 12v battery that drains. The NiMh battery is disconnected by the computer when you shut down. It is the big battery that starts the vehicle, but you need a little 12v power to turn on the computer and get back into functioning mode (the little 12v battery is recharged from to main battery once the computer is on).

The problem here is the small capacity of the 12v battery. All modern cars pull some current from the 12v battery when off. (My Daughter's Jetta was dead after about 2 months of standing idle with a full-sized battery). If you want to avoid the problem, disconnect the 12v battery if you are leaving your HH (or any of the Toyota hybrids) idle for 3 weeks or more.

If this really bothers you there are high capacity batteries in the correct form factor to fit your vehicle, but they are costly. The highest capacity one I have seen is the Lithium Ion "u-charge" (46 amp hours) at about $1000