The defrost water is supposed to evaporate, but if something gets pushed to back of fridge, eg cheese wrapped in paper, it may freeze onto back wall. When auto-defrosting, small amounts can end up going down the drainage hole. After a while, enough gunge can accumulate to start smelling.
Seems to be a common enough problem, happened to me, also a friend and previously on Gransnet. I suppose if you regularly empty fridge and wipe down back walls it shouldn't happen.
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