Getting slightly off topic, but....
My post was regarding placing components or wiring within return air ductwork, which has at least 2 reasons to not be done, as it's a code violation, as well as the facts regarding the production of toxic smoke in the event of a fire. Even plenum cable should not be run through a duct, a plenum is generally the space above a drop tile ceiling that is used for return air when fixed returns are not provided.
Studies have been done where the products of combustion from plenum cable are actually more dangerous than a standard cable, with the difference being a plenum cable is more resistant to the spread of flame than a normal cable, but when it burns, the smoke is more noxious.
I posted my suggestion of locating a SP12F in a somewhat different location than behind the keypad, like adjacent to a duplex outlet or similar.