dead mouse on your plenum.
I really don't know what a plenum is but assume it is the housing enclosing where the flame is, and gets really hot, and if the mouse died on top of it, the mouse gets hot, doesnt burn, but roast and smells. The smell will be blown through the vents.
Is your smell from the vents?
Eventually the smell will burn itself out, like a scented air freshner stick that has expired.
If it is a mouse then you likely have gross mouse droppings and you need to clean it all out. Prepare to take things apart to get to the dropping. Droppings are a health hazard.