It's not unusual for a return duct to be nothing more than a studded cavity. Yes, 2 x 4s on each side, enclosed by drywall. Once the duct 'exits' a wall cavity, then you'll see the sheet metal.
My house (which is 30 years old) has a double-studded return duct on the 2nd floor. It's in the hallway, and it serves the entire 2nd floor.
Just make sure that the furthest 'end' of the duct is properly closed off. In other words, if the return duct continues up into the attic, and the attic is NOT a 'livable space', the duct should be closed off.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!
PPS: If you had a Home Inspection, you can get more info from that source. It should be FREE of charge ...