Can there still be marks left on the windows after a ultra-Pure Water clean when the TDS is 0.00?

Yes, this could be caused by impurities picked up from the window frame, fascia or soffit. The most important remedy is to ensure the window is washed down from top to bottom and is thoroughly rinsed at the end of the clean.

One other possibility is that window frames can be contaminated by soap previously used in a traditional window clean.

It is also important to note when cleaning an aluminium framed window that the Pure Water can draw the impurities from any oxidisation from the window frame. Providing the window is rinsed well at the end of the clean, this should dry clear.