Firstly I don't know if the smell of methane affects a cadaver dog; how was that assertained? Secondly, Apartment 5A wasn't a peat bog, nor did it contain fertilizer.
Yes, methane can confuse dogs.
https://www.murderscience.com/articles/2018/1/29/cadaver-dogs-https://www.academia.edu/602545/Suspect_burial_excavation_procedure_A_cautionary_taleNo, I agree that 5A wasn't a peat bog, lol. However, IF organic fertilizer had been used in the garden, it could equally have confused Eddie - even in the flat (by contamination via footwear, or even by remnant scent wafting in.
At the same time, that's not the sole potentially irrelevant explanation, and I've lost count of how often these have been discussed
ad nauseum.
I have never discounted the possibility that she may indeed have died in the flat as there's no absolute proof of the contrary. On the other hand, IMO, the investigation into potential other explanations as to the dogs' alerts never got further than vaguely asking around if anyone had heard of anyone else dying there.
If ever other potentially plausible and rational alternative explanation had been thoroughly explored and discounted, then I'd have changed my view.
Reality seems to be that Eddie did his best, but could hardly have given a nuanced interpretation to Dad. And Dad had just embarked (or was about to embark) on a solo career.