You'll need a little screwdriver to extract the peg and his surrounding ring.
Here is it, removed from its housing.
Now it's possible to remove the plastic cover.
I've found it easier to start from the side of the seat, pulling to detach the cover.
Here is the 12V battery:
Lesson learned: always keep a screwdriver in the car, in case you'll need to charge the 12V battery while on the road.
Shame on you Toyota: why not just use a tool-less retention mechanism?
Every car driver, hybrid or not, has some story to share about low batteries, so access to the 12V one should be as easy as possible.