Does anyone know of unusual engine trouble with 05' Honda Civics?

I have an 05' Honda Civic, bought it new, maintained normal upkeep schedule all taken to the Honda dealer, normal driving habits. All of a sudden my Honda would not restart for about 30 mins (did this twice). I took it to the dealership & they say I need a new engine and radiator. Then the maint guy goes on to say it looks like it happened b/c some additive was put in the radiator and he goes on to show me how parts of my radiator cap have fallen into the part. He couldnt explain what "additive" and how it got there. I have never taken my car anyone else except Walmart for an oil change over a year ago. Honda last saw my car in June and reported no problems, no check engine light came on, no smoking, or smells to indicate something was wrong. I have an extended warranty, but I am afraid that this was a lemon in the making & it will possible happen again. any suggestions