POH SR 22:

4-21 "Avoid prolonged Slips"
7-57 "Note
When the fuel tanks are 1/4 full or less, prolonged uncoordinated flight such as slips or skids can uncover the fuel tank outlets. Therefore, if operating with one fuel tank dry or if operating on LEFT or RIGHT tank when 1/4 full or less, do not allow the airplane to remain in uncoordinated flight for periods in excess of 30 seconds."(wortgleich im POH SR20, an anderer Stelle).

Das Rätsel um den Kondensstreifen ist damit gelöst.
Danke für die vielen Beiträge.