Why not send the aroma to everyone in the household. Then there will be no question what you are doing.
Ugh. That’s worse than the return grill being in the garage!! I’d write it up for return air grill in a toxic fume area!
I see a lot of these grills on the adjoining tub/shower wall to cover previous plumbing repairs and provide access for future repairs. Also used for insect/termite treatment and access.
Not all are returns.
I tissue test or spark up a fatty before assuming anything.
I personally could after Friday night beer and chilly, take out a whole house load of people…I would call this out as a immediate safety concern from toxic butt sydrom…(this is Humor please see it as such…for the few who have no levity bone)
The aforementioned toxic butt syndrome I suffer from occasionally can be verified by my wife on any given day!
In tears,
Curt
This was an IQ test and yes Barry it was not a return there was a tub on the other side of the wall as you guess was excess to the plumbing. Just wanted to see how far this would go.
You are pretty sharp for a scrawny Texan:p 
you passed some did not.
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Just don’t blow in a wetsuit Curt.
