Hi, When you close any of the doors, the air dispacement needs to be realseased!
basically they flat open when you close a door(s) to let the air out...
You have them behind the rear arch's and poss some in the front doors???
Either way, i have always Insulated, ply lined and carpeted the whole rear of the van.. dont worry about it!
When carpeting, dont forget you need to get access to the rear lights (changing bulbs etc) there are plastic covers that hide the hole, they are genuine vw type and usually on most vans when you buy them.. if not they are quite cheep off the stealers.
That was very observant of you though... i didnt notice them on two of my vans until i removed the bumpers for spraying and a mate slammed a door and i got a puff of air to the face!