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Hello, thanks for posting
Do you have the actual code number for the emission DTC you mentioned?
At that time the engine light must have been on in order to get an emission control fault code. The red triangle is the hybrid system master warning light and can be turned on due to the check engine light because lack of communication between the ECM and the hybrid computer. If the master warning light turns on by itself without a check engine light then the fault is within the hybrid system only and you have to use a scan tool that can access the hybrid system or it won't show any codes. Since the check engine is not on now I assume there is no problem with the engine control system at this time. Maybe the fuel cap fixed that part for now, if the filler neck is damaged and the cap leaks the check engine light will most likely come back on after a couple of trips.
Well, intermittent problems are kind of hard to diagnose anyway, whatever it is seems to correct itself and then turn the light off. Possibly a weak cell or two in the hybrid battery if it's the original. Without a hard fault code it's hard to say.
A lot of problems can trigger the triangle light, take a look at a list possible DTCs for the hybrid system, off hand I'd say about 100 or so from the hybrid batteries to the hybrid computers, powertrain, braking, cooling,etc. I believe there are about 30 or so computer systems that keep the hybrid system operating.