That's one of the details I would like to know more about. If there is no metastatic disease, then how is there detectable DNA from cancer cells in the blood?
There would be a tumor somewhere in the body that is not causing any systemic effects like pain or organ dysfunction. So you wouldn't be going to the doctor because you had a problem, you would be going for routine check up and get the lab done. The cells are cancer cells in the early stages hopefully before they have the ability to metastasize.
Cancer cells when they first appear are from a cell or cells that have changed to cancer cells. They were a kidney, liver, lung ... (pick your organ of choice) and now they're cancerous. It takes time to go from a cancer to a metastatic cancer. First they form a primary tumor then over time they breed in the ability to come apart, invade local tissue and slip into the blood stream or lymph system. Over time again the cells that have slipped away have to land in a new spot and take root to form a metastatic tumor.
The idea behind the test is that the cancer cells are putting out DNA sequences that say I'm a cancer and I'm this kind of tissue. So if I routinely test I should be able to tell you when you first get a cancer before it has become metastatic. By being able to tell what the tissue type is we can focus an MRI on that part of the body and pinpoint where the cancer is. This could mean we could surgically remove it at a point in its development that the surgery would be a full cure (like doing a colonoscopy and removing a polyp before it becomes cancerous).