There has been a thread on deleting a source that is a link to an Ancestry tree that cannot be found. Some notation about this link should be kept, since it might be a clue to find information.
I just encountered one of these, as the only "source" in a profile. The Ancestry tree could not be found. I signed onto Ancestry and did a search for the Profile Manager (by name). Once I found them, I looked at their trees, and found the person in the profile. That led me to the Ancestry trees used for the 'source' of that person. By looking at those dozen or so trees, there was one or two that had a record source. (The record source, btw, does lead me to suspect that some of the data in the profile is incorrect; more research is required however.)