M
Mickymoody
Interesting theory Mickymoody but actually, as far as I'm concerned, not only do I not have any wall paper in my home, but my problem was a twelve month a year problem. The rooms affected here have the original plaster of the house.
We have a consumer programme on the TV over here, that has highlighted this fishy smell issue, and you being based in Canada, I realise that you are not one for being a nation of wallpaperers, hence I mention the facings, as we used to get a couple of maniacs that were Canadian based on our TV, that did DIY, dangerously! So, as I understand it the basic house is built, of wood, then a fascia is built around it for appearance? It may be that damp has got into that space, to cause localised rot, plus the degradation of any glue used in the manufacture?
Also the roof is wood based, with felt liner overlaid? So maybe a tear that is letting damp in? Canada generally is cold/damp to my mind, never having been there, so the problem being constantisn't a surprise, but a warm snap, as you mention, the issue was resolved temporarily.
Just throwing ideas at you, as it seems like you are at the end of your tether regarding this, and how to prove me idea, or how to rectify it, apart from ripping your house apart, unknown..