This is the key piece of information, and why I was asking. When you first switch to HV the engine will come on but it first goes into a warmup period where it will just idle as it warms up. During the warmup period the engine will produce little if any electricity. You can see this by observing that your EV range will continue to decrease while the engine is warming up. After the engine is warmed up then it will start producing electricity. If the car is stopped at a light after the engine is warmed up the engine will usually shut off. In most cases that is.
Your situation was different because you were climbing a slight grade during the engine warmup period, so it used up more than the usual amount of battery power during that time. Even though you had plenty of charge (80% as you said) the way HV works is that when you first activate HV it creates a "set point" at whatever SOC you are at when you activate HV. From that point forward as long as you remain in HV mode the engine will come on as needed when the SOC drops below the set point. In normal situations when you stop at a light you would at the most be only slightly below the set point and so it won't turn the engine on while stopped. But in your case I think it's the fact that your SOC would have been way down due to the slight but long upgrade, again during a time when the engine was contributing little if anything due to it being in warmup mode. So in that less than common scenario it's not surprising that even while stopped at a light the engine remained on to continue charging the battery. It is however slightly surprising that when you turned of HV while sitting at the light that the engine didn't turn off. But I suspect that's because the engine was still not fully warmed up, and since the SOC was pretty far below the set point, it decided to keep the engine running to charge the battery while it continued warming up the engine.
There is no official documentation of any this by Honda. I suspect that they consider this level of intricacy about the operation of the system to be proprietary information. So we can only infer things from what we observe, and by noticing what the usual patterns are. Then if things work differently in a particular case we can try and think what may have been different this time, in this case a slight uphill climb during engine warmup ending at a stop light. Now if you see it happening in other situations like flat driving and a fully warmed up engine, then something else is causing it.
If you are interested in the workings of the Clarity you can get an inexpensive OBD-2 scanner and a free bluetooth app for your phone, which will show engine RPM and coolant temperature among other things. Of course the great thing about the Clarity is that you can also just drive it and not think too much about these type of things, unless you find it interesting.
A good thread to read about an OBD-2 scanner and free app is located here:
(UPDATE - This thread was started when the only known way to check battery capacity was through the dealer. The general information below about battery capacity is still valid, however starting in post #6 there is information about checking battery capacity yourself using an inexpensive and...