If it's in the return duct, then air isn't going out, it's coming in.
I agree with the poster that said with the damper, it looks like a way
to use the blower to bring fresh air into the house when the damper is
open and only the blower is running, not furnace/AC. I'd get rid of
that setup pronto. Hopefully the damper was closed, cause if it were
open, you'd be sucking in a lot of outside air while trying to heat or
cool. And it looks totally impractical, as to fool with it, you have
to go into the attic and it's very easy to leave the damper open and
forget about it. And for what benefit? If you want some fresh air
that way, just open a window!