cells turn cancerous all the time and usually the body eliminates them without issue. this blood test is still going to hit all sorts of "false" positives, where it detects some cell that has gone cancerous that would have been eliminated anyway.
I'm far from expert but I don't think this would be a large concern. My understanding of CTCs (circulating tumor cells) is that they are shed by a growing tumor. A random weird cell doesn't seem likely to be turned up in a test like this. You need some amount of quantity to set off the proverbial alarm bells
