....by an otherwise perfectly innocuous and harmless object?
Very strong odd smell, sort of like damp, in one corner of the living room for a couple of days.
Hands and knees on the carpet, feeling for any signs of damp, check around the radiators, check the walls for signs of damp patches. Nothing.
Start moving furniture, is it something one of the cats has brought in that's crept under the settee or behind a cabinet and died. No, nothing found.
Removed the part-finished box of After Eight Irish Cream chocs (and they were pretty eughh, didn't compare in any way with the good ole mints!), the smell disappears.
Wouldn't have believed they ponged so much and so unpleasantly if I hadn't smelt the results, IYSWIM!