It's hidden in plain sight .
Look at the top of the housing that the main filter is attached to.
You'll see the large bolt that holds the main filter in place.
Towards the engine valve cover (to the left if standing in front of the car) you'll see this black "disc" with a kind of a knob shape in the middle of it, THAT'S the prefilter. It's held down with a kind of a V or C shaped bracket, which in turn is held down with a bolt.
If you replace it, pay attention to the fact that the original has a rubber o-ring that seals it, usually the new one won't have an o-ring with it, you should buy a new one.

Gilly