This moment from S2E8 ("Better Call Saul") has been nagging me for a while. Saul enters Walt's classroom:
Walt: How did you find me?
Saul: We should talk about that. It should be much, much harder for people to track you down. My PI charged me for three hours, so I seriously doubt it took him more than one.
As we later find out, Saul's PI is Mike. But how did Mike find Walt? Keep in mind that at this point, Saul still thinks Walt is Badger's uncle; their only interactions have been those connected with Badger's arrest.
In addition, neither Gus nor Mike has made any appearance in the show yet.
Any ideas? I remember being a little curious when watching this as to how Saul could have found Walt; to me, at least, he seemed to have covered his tracks sufficiently.