Had this on a house in Lyme Regis that I had cleaned many times. TDS was around 2-3 and the house was cleaned and rinsed as usual - had spots on most windows, but more on the south facing side and steaks at the edges of many. No other houses cleaned on the same day had any issues and I went back, re-cleaned and all was fine.
I'm wondering if there any other external factors that come in to play: excess presence of salt due to the proximity of the sea, unusual atmospheric pressure, contaminates carried by weather fronts or just general weirdness ?
Eight years now wfp with nearly 100 % houses cleaned with no issues - just the occasional weird one.