Grimes report talks of scent building up in an apartment closed down in a hot climate:
What we have to be able to understand in a situation such as this is in a hot climate with the apartment being closed down, the scent will build up in a particular area. If there isn't a scent source in here, i.e. a physical article where the scent is emitting from, any scent residue will collect in a particular place due to the air movement of the flat, the apartment and what I would say in this case is that there is enough scent in that area there for him to give me a bark indication but the source may not be in that cupboard, the source may well be in this room somewhere else but the air is actually pushing into that corner. But strong indication and I would say its positive for things that he is trained to find, which will be part of a separate debrief.
It sounds plausible to me the scent was emanating from Mrs Fenn's and somehow seeping through the floor/walls/pipes etc.