When and how did Obi-wan find out that Vader was still alive (after the events in EP III)?

Hey, I would've posted this topic in the "Revenge of the Sith" board, but they aren't allowing new topics, and it is relevant to the OT board as well.

Anyway, here is my question, and please note that this includes "Revenge of the Sith" SPOILERS:




After the events in Episode III took place, when and how did Obi-wan find out that Anakin/Vader was still alive? It's not obvious at the end of Episode III that he finds out, since, as we know after seeing the movie, he leaves Vader/Anakin for dead on Mustafar.

Does he even find out that Vader is still alive, until he sees him on the Death Star in "A New Hope"?

After Obi-wan goes to Tattoine (at the end of EP III) and drops off baby Luke with Owen and Beru, it's presumed that he isolates himself for the next 20 years (until the events in A New Hope). However, when he says to Luke (in his hut on Tattoine) that Vader hunted down and destroyed the Jedi Knights, it's not clear whether he's referring to the Knights Vader destroyed before his fight with Obi-wan on Mustaftar, or the ones he destroyed after he had to put the life-supporting armor on....

 

This question is for the LIT forum. The EU will answer it.

 

who knows, this doesn't belong here anyway