I notice stuff in my urine sometimes ... I mean, I don't pay that much attention, but I can think of times where I have seen stuff in it that was more than just urine. What it was, who knows? Urine isn't just used water - it's stuff that has gone through a nice fun process that goes on in your body. Stomach, kidneys, bladder, etc ... sometimes there will be more in it than just urine.
Again, remember - the doctors have examined you, and found nothing they are concerned about. Certainly not cancer. So you can either go on convinced you have it and prepare to die (although you'll be waiting a long time) or you can accept the diagnosis and move on with life. I mean, if I just think about when I was sitting around examining my feces because I was so scared of having blood in my stool ... well, it's just embarrassing to think about it now. It seems obvious - if you're sitting around examining your body's excrement, there's something wrong going on, but not in your body, rather in your mind. That's something I eventually saw with my own stool examining experience.
I totally get where you're coming from, and don't worry - you're not annoying me, or probably anyone else. This is a forum for people who habitually worry about their health so of course you're going to have people posting regularly. I just want to encourage you to listen to the doctors. If they were worried about cancer, they'd be pursuing a course that helped you get over it. But after all the tests they have done, they aren't. So neither should you be. :)