I think hasarad's hunch is right about the 12V battery. Per OM p457, "You may occasionally find that it takes longer than usual for the vehicle to start moving when you select (R), release the brake pedal, and/or depress the accelerator pedal. This may occur when the High Voltage battery level is extremely low, and does not indicate a vehicle malfunction."
Also, how were temperature conditions? From OM p458, "When shifting gears in extremely low temperatures (−22°F/−30°C), there may be a short delay before the shift is indicated in the display."
Were you eventually able to keep it in gear to drive it around a while? The HV battery recharges the 12V battery via DC-to-DC converter (vs alternator), so the same principle of driving around and/or keeping the car running could help build charge in the 12V battery if still healthy.