Ah, that's my mistake. I thought your readout showed actual gas being used. If it's not, then you're probably just dealing with the same glitch as others.
I wonder if the Volt uses the ICE to retard forward movement when the battery is at capacity, since there is no need to regenerate extra energy while braking. That might make the system register ICE use, even though no gas is being used. Traditional cars can be used to compression brake in a very similar way.