The car engine will fire up, then cut out which will then deplete the battery.. and then the engine will fire up again. The car hasn't got a temp gauge, so you won't be aware of what over heating may be occurring, until the warning light comes on. Idling for more than a few minutes is not good for any car, regardless of hybrid configurations, which is why a lot of cars now have the stop/start feature (which isn't just an economy feature).
Plus, it is bad for the environment.