I don't recall seeing the walls like this before, so I don't think it's just accumulated soilage. If it's wetness, what design flaw causes the windows to do that? Today was very hot and muggy so it's likely the AC was running all day.
The building is in a place (Halton Hills, Ontario, Canada) where the relative humidity is about 100% in the summertime (see part Climate in this https://www.britannica.com/place/Toronto)
The building must be heavily air conditioned due the numerous medical businesses which do not stand uncomfortable indoor conditions. As already said by others, it's well possible that those single layer glasses are wet all the time in the summertime. The continuous water flow condenses dirt just below the windows which have nothing to keep the wall dry.
I guess the the tiling below windows has holes which leak out a substantial amount of indoor air. In the summertime the walls get wet also because the wall near the holes are cooled by the indoor air. In a cold weather the walls get wet because there comes out warm indoor air which carries out water. See this Google Maps image:
The image is shot in march (=early spring). The windows looked the same in all sides of the building when I drove around with Google Maps.
When looking older versions of Google images one can notice that the stain seems to be much darker in the autumn - like some midew has developed in the moisture on the stones during the summertime. I guess it tears off during the winter.
OP states that the staining is not always present. here is a possible reason for it:
If the building is air-conditioned and has single-glazed windows, then the outside of the window glass might be below the dew point temperature of the exterior air (which varies from day to day) while the AC is running. This would cause occasional condensation on the outside of the window glass, which would then run down and wet the building walls under the windows.
To add to the other answer:
When ledges protrude out from the wall then the water, and dirt, falls away from the wall surface. Also, the underside of the ledge usually has a drip channel or groove to stop the water running back onto the wall.
So flush window ledges have this problem. Windows with protruding ledges can have the opposite problem where the wall under the ledge actually stays cleaner than the rest of the wall.