That is an excellent idea, thanks! Unless the dealer has some better idea after watching the video, I will propose your plan to them. If I pull the HVB disconnect every time I park the car for 2 or 3 weeks and the problem stops happening, that would definitely seem to isolate the problem to "not the battery pack itself."
The dealer checked for error codes (as did I, although I think the dealer's equipment might be able to access more codes than mine can) and did not find any codes stored.
I do understand that battery SoC estimation is not an exact science, but I don't think that is what's happening here. Going from 40-60% to 0% would be a pretty large gap in estimation, and, if that were happening, I would expect the car to behave oddly either before or after the jump (depending on which state of charge is actually correct). What actually seems to happen is that the car behaves completely normally "before" the jump to zero, and after the jump to zero, the ICE runs constantly until the indicated charge is back around 50% even when conditions would otherwise cause the ICE to shut off. It seems to me that this indicates that the HVB charge was actually low, and that it is actually being significantly recharged by the ICE.
The other thing as far as this being a normal glitch goes is that it has happened 8 times in the fewer-than-three months that I've owned the car. That comes out to every one and a half weeks on average, which nobody else seems to be experiencing.
This is probably stuff that most everyone here already knows, but here is why I am concerned about this problem, beyond just "I don't think the car is supposed to do that":
1. The HV battery and many of the other HV components are expensive parts which are, for now, under warranty, but will not remain under warranty forever.
2. Hybrid vehicle HV batteries are designed to be charged and discharged carefully and within strict limits by the vehicle's energy management system. When the dashboard on a hybrid car indicates that the HV battery is at 100% charge, the true state of charge of the battery is considerably less than 100%. Likewise, when the dashboard indicates that the HV battery is at 0% charge, the true state of charge of the HV battery is considerably higher than 0%. Maintaining this narrow range of charge states is one of the reasons that the HV batteries in hybrid vehicles last much longer than a typical cell phone or laptop battery. Discharging a hybrid HV battery beyond its designed-for state of discharge (or under conditions beyond its design specification) can easily do permanent damage to the HV battery which might not become apparent until well after the battery is no longer under warranty.
3. In the 2012 Ford Fusion Hybrid, the HV battery is the *only* power source capable of starting the car's internal combustion engine. This means that if the HV battery is discharged beyond a certain point, the car will not start. If this happens, the car must be towed to a Ford dealer, and most Ford dealers do not have the necessary equipment to recharge a drained HV battery on hand. The Ford dealer will have to request the necessary equipment from Ford corporate (or another dealer), which will cause extended delays and cost me a lot of money for alternate transportation during the delay.