A rusting fastener was one sign that seemed to suggest water leaks around a window. But when the author was called in to perform a window leakage test, no leaks from the window units were detected. The rust stain is at the level of the bottom wall plate. The apron below that is covering the slab edge.
The leaks that rusted the fastener in the previous photo was caused by water leaking under the bottom wall plate. The cause was likely due to poor flashing of the wall recess below the window.
A thermographic view of the opened wall section (shown in the previous photo) shows extensive wetting of teh slab (visible as cooler areas in purple).
We tend to over-focus on rain coming from above, but runoff from rain falling on other areas of a site can often be even more devastating. Water running down this slope, for example, will be absorbed by the brick veneer, which will hold water in the wall and eventually cause microbial growth and decay of the structure behind the brick.
In addition to the siding being too close to grade here, the gutter downspout is dumping water on and under the building.