Charles:

The "Check Hybrid System" message that you got on the multi-information screen each time, suggests to me that there may possibly be an intermittent electrical leak from the NiMH "hybrid" battery to the vehicle's chassis. This would also explain why the high-voltage battery was severely depleted each time. The 2007 Camry Hybrid's "Emergency Response Guide" says (on page 13):

"A ground fault monitor continuously monitors for high voltage leakage to the metal chassis while the vehicle is running. If a malfunction is detected, the hybrid vehicle computer will illuminate the master warning light <!> in the instrument cluster and indicate "CHECK HYBRID SYSTEM" on the multi-information display."

Have the technicians considered this possibility? The dc-to-dc converter might be one suspect component, but there are many other possibilities for leakage. Just an idea, since they seem stumped ...

Stan