If the marks are inside then they've either been there from the begining OR the unit has blown, the water staining is likely from dried condensation inside, at 5 years old its not very good at all, I'd be calling the installers back and claiming under the 10 year guarantee assuming it covers units for that long, some 10 year policies only cover the frame for 10 years while the glass is 5 years and handles hinges and locks can be as little as 1 year, we call that a 10/5/1 and most of my work was under 10/5/1 policies however some companies offer 10/5/5 and some even 10/10/10. If the company is no londer in business then your IBG will kick in and take over the remaining balance of the 10 year warranty but the same rules apply re the length of time stuff is covered for
