Hi all. First Post, sorry if telling you how to suck eggs.
I have known of a problem similar to this with another cleaner. After leaving the house he cleaned, a neighbour started some building work creating a lot of dust.
This stuck to the beads of water that were left on the glass.
Make sure there is nothing to contaminate after you have left - dusty paths, even someone having a bonfire - smoke will spot your windows also. In such a case it is always a good idea to blade windows too.
Paul