The wall suddenly smells like marijuana and stale beer.

For understanding. It is about the ceiling. So, the sloping roof. Or the wall (knee wall) below the sloping roof. What is behind it? Is that known? Cavity, roof insulation?

I also remember the thread :p

I consider the smell of decomposition possible. It can be quite pungent. My parents once had a cat that occasionally dragged mice into the basement. Every now and then there was a rather peculiar smell, and eventually one found a decomposing little mouse under a cabinet or behind the washing machine.

But I wouldn’t have associated it at all with stale beer or even "grass," that smelled different.
How do I know this... ehh... a friend told me... ;)

The question is whether you have any chance of finding this out without opening the wall.

For clarity. It’s about the ceiling. So the sloping roof. Or the wall (knee wall/collar wall) beneath the sloping roof.
What is behind it? Is that known? Void, roof insulation?

Aha, a sloping roof in a house about seventy years old (thus around 1950). At that time, the battens for the roof tiles or shingles were usually attached to the rafters (layman's term: roof beams) on the outside, and on the interior side, due to post-war reasons, very individually some boards or panels, and nothing between the rafters. Then in summer it is a sauna and in winter a refrigerator – if not, the construction will no longer be "original" and someone still alive would know what was done. Is it just odor, without moisture and without material discoloration? – would you see anything if you photographed this sloping roof (I suspect not, otherwise you wouldn’t first be sniffing around looking). Speaking of sniffing: are there pets (- how do they react to the smell)?

But I would never have associated it with flat beer or even "weed," it smelled different. How do I know....uhh...a friend told me...

We used to have mice and martens on the roof from time to time but haven't for a long time now.‍♀