There have been a spate of news articles since the New Year speculating on a run. The Hill is not an obscure website. I just saw an article two days ago on this. The Dems have some sense that the announced candidates are weak, at least without Biden officially in the field, and are trying to come up with other names.
For what it's worth, Trump's people fear Beto the most. They think he has charisma and obviously believe policy stances are not foremost in determining who will become prez.