In the summer I started advertising in the local community "rag" and took on about a dozen folk, most of whom were done by a disappeared traddy guy. Some were "blown" and it is simply those that are ten/fifteen years old and mainly the openers, especially ones that face the sun and get the heat and rain (cool) alternately over the years.
Wfp might "show it up" but the prob was there already and is due to poor manufacture, vibration when closing, poor fitting (base of unit in bottom of frame and getting wet instead of being up on a hidden spacer) or age (expansion and contraction in heat and cool).
I have a husband and wife team called "windowcare" who I pass all this on to and every so often I get a crate of wine of beer - which is nice!
There is no way wfp will get water between the panes - it has to get past the seals between the pane and the beading and then the glue that holds the unit together. If the window units are that bad then they will rattle in the frame anyway!