Aluminium foil on windows: what is the residue, and how to remove it?
Finally I used a glass scraper. Bleach didn't help, and I didn't dare to use even stronger caustic soda. 

Fuller details at the answer I wrote at the DIY SE site: https://diy.stackexchange.com/questions/151160/how-can-i-remove-residue-left-after-removing-aluminum-foil-from-my-windows#answer-152162