I would be curious about a scientific explanation of this, not the over-simplified version that is released to the public.
I’m thinking that early cancer is localized, so there really would be nothing to detect elsewhere in the body or in the blood. But a cancer that has begun to metastasize would be shedding cells, and some of them could make it into the blood.
I’m guessing that the DNA markers they are looking for are epigenetic alterations, not changes in the sequence. It could be both, I suppose.
Anyway, I can’t really comment on how likely this is to really work or on how many cancers it would detect, because the article gives so little solid information.
If your using the test to detect early cancer then there would be no metastatic disease. The test would tell you the type of tissue the cancer is located in: Lung, Liver, Intestine. It would not give you an exact (x,y,z) coordinate/
If you use the test in metastatic cancer it would tell you what tissue the primary tumor came from.