If I turn off the thunderbolt phone at 25% remaining, you would think battery would have something left still. Makes me think it is something that is a hardware design flaw, and something software or firmware updates may never fix? Not a good phone to travel with, always have another means to call if you think turning if off will help, it does not.
Just like many appliances in your home. Just because they have been turned OFF, doesn't mean they are not drawing power. To truly turn off, take out the battery (unplug from the outlet).