the pipe runs from the front, driver side, of the engine across to the bottom of the aos on the other side of the engine. it weaves through the driver side intake manifold in the process and is a general pain in the arse to work with and around.
the aos vents both sides of the engine; this pipe accesses one side and the aos bolts over the access on the other. the aos outputs to a pipe that vents to the intake plenum after the throttle body.
Stig, what year is your Boxster? Mine is a 04 S this is my AOS and piping to it.
That pipe you show reminds me of something on the rear fire wall behind the drivers seat? I'll see if I have that view. Did you remove your engine?
Is that possibly your brake booster vacum line that attaches at the front of the left (drivers side) intake manifold?
In my pic I unscrewed it at the flange instead of using the proper disconnect coupling, PITA.
On my car the AOS feed from the opposite head is a complete ribbed flexible tube, attaching at the bottom side of the AOS, earlier models may be different.