L.A.
I've heard that this can be true with developmental milestones such as walking. You can certainly re-introduce sleep training. We did Ferber months ago.
We had a few early wake ups this month (DS started walking on the 2nd). If he was up early, I waited 5 minutes (most of the time he settled himself) at the 5 minute mark, if he wasn't asleep, I walked in stroked his back, and laid him down again, he was back to sleep b/f I made it back to bed. It's happened once or twice, but because we didn't make a production of it, it's been nipped in the bud.