Every once in awhile, something gets lost in the dark and dreary corner of our fridge.
And it smells.
And sometimes, I think even a truck load of baking soda wouldn't help. Which is when I pull out the big guns: Charcoal. As in the charcoal you would use to grill outside. Just plain old, non-"enhanced" charcoal. (In other words, don't put charcoal that was bathed in lighter fluid in your fridge.)
You just put a couple of briquettes in a bowl in your fridge. Leave them there. If the smell is bad, I'd put a couple on each shelf. They will absorb the odors & help keep your fridge smelling good until you find the culprit.
You can use this all the time instead of baking soda for odors in your fridge. Just change the briquettes out once a week!