This question came up not too long ago, so may be you can find it back. There were several answers. I understand they only check the large bowel and not the small intestines.
I had a a virtual colonoscopy (with a CT scan)and they said there were a few small spots they could not see, but for the rest no problem.
If we all had as much fecal matter as John Wayne and Elvis Presley and we did the bowel cleanse, we would all loose between 10-40 lbs. I have not heard about that yet.
You could type in the search box, right top at page "fecal matter" and see if you can find some old messages. You try different words like "colonoscopy".