I personally feel like this question is off-topic since there seems to be no reason why this question couldn't refer to a mundane, real life scenario. There is nothing for us to add that chemistry.SE couldn't do better.
I'm not 100% sure though and feel like it's valuable to discuss these edge cases more explicitly, so we can make a more confident decision the next time around. I'm also not entirely sure if this specific question could be rewritten to be more on-topic.
I think it's off-topic because it's not about the world. I don't think its being a small detail is the issue; worlds are made up of small details and it should be ok to ask about them. But this doesn't seem to ask about building a world.
A question that asked "I want smells to be able to linger for several months; what properties of my world would cause that?" or "if it's implemented by X, what other properties would that have?" would be about the world and thus on-topic.
I think this is off topic for a number of reasons:
It's not speculative - as you say a "grounded in reality" site can answer it
It's too small scale - world building can theoretically cover things as small as a village or building but this just feels too small
It's not about building a world, it's about research for a plot point within a world.
I agree it's definitely in the gray area though. It's not completely off topic, but I think it is enough off topic that it crosses the line.
I don't think this question can be re-written to fit as there is no way to make it about shaping the world. The question isn't asking about the world-shaping effects of lingering smells, or how to change how long they linger for, or even about the effects of scent on creatures. It's asking for a simple real-world forensics figure.