Sudden fail of CC + no EV mode + hyper-regen braking = B Mode (Solved)

1. After stopping at Florida welcome center on I95 this afternoon, the cruise control would turn ON but I could not set any speed no matter how I tried to use the stalk to cancel, set a speed, or nudge speed up or down. Tried 3 times turning CC off and on, and I was still was unable to set a speed and have it engage.

2. When coasting to a stop at the next exit, I saw a much stronger regen level on the Hybrid System Indicator than I have ever seen while coasting before. Normally The level of regen on coasting is just to underneath the "G" in the "CHG" label, and now the regen was between the C and the H, plus I heard a low rumbling or whirring noise from underneath the car which for exactly the period of the extreme regen braking. The road was dead flat, and the HV battery was not full.

3. Continuing at slow speeds on local highways, the Prius failed to engage EV mode even when I was well under 40 mph, and the extreme regen braking continued to re-occur whenever I let up on the accelerator pedal. HV battery temps stayed in the 35-37 degree Celsius range and battery fan motor stayed at level 1.

The conditions were that the outside temp was 73, we had the AC off but the cabin fan turned up fairly high, the ICE coolant temp was 195 degrees F, the HV battery temp was 35 degrees C, the Battery Fan Motor was at level 1, and there was no indication on the MFD of any malfunction . However, we had started this morning In the 50s in North Carolina, so the lower grill was blocked.

I stopped, shut the engine down completely, opened the grill blocking doors, and continued to drive, whereupon ALL of these odd symptoms simultaneously vanished.
So , folks, what is your best guess on the exact point of failure?
If you think it is a computer, exactly which computer would it be that controls this precise set of things?

FWIW, the 12-volt battery is the original 4-year old Toyota battery, the car has 64,000 miles, and I have had zero other problems up to this point.
Thanks in advance for any insights!

 

Damn you guys are good!
JimboPalmer wins the prize!
DW had accumulated Florida visitor guides and an official plastic bag to store them all in. After I backed out of the parking space at the welcome center , she hung the bag over the gearshift lever in such a way as to precisely pull the lever to the right and backward, which would shift me unbeknownst into B or braking mode (see picture). I have never used B mode and thus had no idea that you could drive the car forward whilst in braking mode, hence my puzzlement, and stopping and restarting the car would make me automatically put it back into D mode where it would perform normally. DW says "Thank You" to you guys.

 

Er, those are the plexiglass doors that slide out to the side to block the lower grill, or slide together in behind the front license plate to open airflow to the lower grill ((Not Toyota OEM). The doors slide in slots in sections of an aluminum picture frame that was built to fit the opening, and are secured in either the "open" or "closed" positions by the small bungee cords you see dangling from each one in the picture above. I left the doors in the half-open position as they are hard to see when fully open as they look like a window, or when fully closed as they are behind the license plate. The far left and right sides are also covered with fixed plates of Plexiglass to create a smoother contour for non-turbulent airflow when the doors are in the closed position, as they almost always are except in the summer months or when climbing mountains. So far no signs of corrosion or other failures in this rather simple system, and it makes the blocking or unblocking of the grill a neat, 10-second task. Cheers and thanks for the ideas on my problem!

 

Thank you Tideland!
I went back to reread my owner's manual, and B mode can only be engaged when you are in D, which is why I did not see it when first turning the car on. Also, although the manual says that you cannot engage B mode when you are driving with cruise control, it did NOT state that when B mode is activated, the cruise control cannot be set, and the completely bamfoozeling thing is that the little cruise control icon on the MFD did in fact illuminate, making me think cruise control had been activated, but it would refuse to accept a "set speed" command and I never got the SET illuminated under the cruise control icon. That oddness of the failure is what made me consider I might have an overheated computer chip that was misperforming in a very odd way. But thank you all for your consideration, and as the Bard put it, "All's well that ends well!"