I was changing out my air filter and in the process of cleaning out the air filter box I took a closer look at the source of this funny square patch of cleaner air filter media. This clean patch sits over this small rectangular box in the corner of the filter box. I've seen this odd thing before in previous air filter changes but what I noticed this week was that there was a hole to the atmosphere at that corner as you see with my image in an image below where I'm basically probing my finger from the back of the hole into the body of the air filter box.
What's this for exactly? This hole is surely by design, what purpose does it serve? Am I missing some sort of foam plug or something that should be plugged into that hole? Is it like a vacuum modulating area for the turbo of the vehicle such that a maximum vacuum is clipped by the presence of such a hole? I should note that the portion of the air filter box that's shown is the dirty air side which is the bottom of the box. The top receives the filtered air that goes first to the MAF sensor and ultimately the intake. The vehicle in question is a 2005 Volvo S80 5 cyl 2.5L Turbo engine B5254T2