This is a kitchen-related question

[Warpy]

9. Activated charcoal will do an odor absorption job

As for where it's coming from, obviously there's something under or behind something else that has accumulated 12 years of scuzz. The best idea is to strip everything out of the fridge, every shelf and every drawer, and examine all the cracks and crevices you find, cleaning them if there's anything at all showing up. Break out the pipe cleaners if you can't get in there with a folded paper towel.

Then, while the fridge is light and empty, move it out and check behind and under it. Chances are there will be nothing there and that you've found whatever it is in a shelf race or drawer support, but it's one of those "while we're at it" things.

Use the activated charcoal to soak up any residual reek and you should be fine.

Baking soda really doesn't do much.