I was diagnosed with rectal cancer in 2005 by a colonoscopy. I did have symptoms, though it wasn't obvious at the time, since the blood in my stool was attributed to hemorrhoids. So, formally, my colonoscopy was for screening. After I was treated for rectal cancer, I became interested in CR cancer, and have read on line of a fair number of people who were diagnosed through a screening colonoscopy but who had no symptoms at all. It really happens, though not often (probably because not all that many people get screening colonoscopies starting at 50, even though that is the standard of care).
The fecal occult blood test is very useful, but it is not as good at detecting CR cancer as the colonoscopy.