As per subject heading, I have a mild but persistent smell in my
fridge, as though something in there is bad - only problem is, there
isn't anything in there like that.

A few days ago I removed everything and thoroughly cleaned the fridge,
then checked (and cleaned) every item before returning them to the
fridge. I never did find the cause, although I did suspect an egg
carton that has been reused many times (that never went back into the
fridge). I also checked the eggs for any bad ones. Nope.

Yesterday I tried an onion halved, then some vanilla extract.
To no avail...

The fridge is in good condition and about 5 y/o now. Always been kept
clean. There are one or two vents inside the fridge I haven't
investigated as yet... but will after posting this.

Does anyone have any ideas how to get rid of the smell?
No, I'm not buying a new fridge, so let's not go there