Past "memories" have shown Dolores standing around dead bodies; presumably this is a vision of the oft-alluded-to incident that occurred in the park 30 years prior to the present-day events of the series, and the third episode's final beats, in which Dolores pictures the Man in Black, shoots a bandit host, suffers a shot to the abdomen herself, and then flees back to William, all seemingly in the same stretch of time. If Dolores is connected to The Incident -- and come on, she has to be -- it's likely she went through the maze years before hearing about it from Bernard in "Dissonance Theory."
The Las Mudas moment bundles all the evidence together. William seems to have led her there in the past, intentionally or not, and that experience is what's drawing the older William (the Man in Black) back to decipher the maze game. It can't be just a coincidence that the Man in Black stopped in Las Mudas before continuing on in the game. And if what I'm detecting is true, present-day Dolores is hot on his heels, her memories giving her an advantage.