[keraf]

A sibling comment mentioned that it can detect smoke, so my guess is waste disposal, aka burning trash in your backyard. Some can produce quite a lot of smoke.

I did a private pilot license in Africa, the biggest "plus" was that I always knew the direction of the wind on the ground by looking at all these fires. There was never a time when I didn't see smoke unless I was flying in very remote areas.

[alienthrowaway]

Why did you assume they are burning trash instead of log fires? What was the primary fuel for cooking and heating for poor people in the part of Africa you got your PPL?

[keraf]

It's not an assumption, it's commonly done in some parts of the world that don't have proper waste management. What happens after you put trash in a bin and there's nobody to collect it? You have to get rid of it somehow. Out of many options, such as throwing it in the sea, burring it in the ground or simply burning it, the latter is usually done because it's easier and quicker.

Heating isn't really an issue in the part I lived (warm all year round, even in the night) and the smell of burned plastic is quite distinguishable, even at higher altitudes.