One theory is that, as the windows heat up in direct sun, the pressure builds up in the window and the glass is flexing ever so slightly... just enough to trigger this effect.
Take a flashlight and hold it face to face against the inside of the inside pane, near the edge. If you can see the reflection of the bulb on the inside of the outer pane, make a mental note of it's position. That offset between the flashlight and the reflection is a function of the space between the panes.
Now, slide the flashlight to the center of the inside pane. Where is the position of the reflection? Does the reflection remain in the same relative position as near the edge, or does it move?
The amount of movement reveals an inconsistent void, indicating a deflection between the two, theoretically parallel, surfaces.