I've noticed the same thing, but it's not really different than any automatic transmission car where if you don't press hard enough on the brake the car will begin to move by itself. In the TCH, that motion could come from the motor (or even from the ICE if it happened to be running while stopped). It's not really that big a deal anyway. Since the car is hardly moving at all, it's hardly consuming any power from the hybrid battery. Seeing the power flow on the display looks worrisome, but remember it only shows which direction power is flowing, not how much is flowing.

The best solution is just to press the brake a little harder. As WVGasGuy noted, using neutral is not recommended because the hybrid battery is not charged when in neutral. There's a section on that in the owners' manual.