When we go purchaesd our 2007 CPO X3 it had been sitting at the dealer for a while. Not long after we got it I popped the retainers on the cabin air filter just to check the drain holes since they are one source of creating SAV swimming pools. I found what had been a mouse nest at one end of the tray so made it go away.
Flash forward 22K miles and the to the third oil change I just did and my decision to put in a new air filter. The old filter was really dirty and underneath it was a really huge pile of grass, leaves, and pieces of pine cones - virtuallly everything necessary to make a nice home if you are a mouse or chipmunk so made that go away as well.
So, moral is: if you SAV is going to live outside like ours does, best to check under the filters at least once a year. For the air filter I did loosen the clamp to the mass air flow sensor but didn't need to disconnect it from the air box cover. Just undo the 4 snaps and you can open the cover enough to swop filters and vacuum the box clean.