Strange filter

During an inspection today I stumbled on a strange plastic filter mounted high on the wall in two closets. The filters consists of a plastic cover at the interior and exterior wall with a piece of filter material between. Nothing mechanical appears attached to either. The filter covers are about an 1-1/2" X 12" long. Can anyone help identify these and what they are for? I believe the brand name on them was Alder.

Sounds like some kind of return air filter, or a passage way through a wall, with a filter of some kind. I’ve never heard of that filter name though, maybe someone else has seen one…
got a picture?

Yep I do not understand not taking pictures, as since if we do not know,imagine the poor client.