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In Bereishit 14 Avraham rescues Lot after the battle of the four kings and the five kings. At this time Lot is living in S'dom, and it seems reasonable that Avraham knows that. (Lot was captured when S'dom and 'Amorah were attacked, and Avraham interacts with the king of S'dom.) In chapter 18, God tells Avraham what He plans to do to S'dom and the other cities of the valley and Avraham "bargains him down" to saving the cities if there are ten righteous people. In the next chapter, the messengers rescue Lot and his family, who go first to Zoar and then into the mountains. Then God destroys everything in the plain and Avraham sees the result (19:27-28).
We don't hear from Lot again, and the last Avraham knew, Lot was living in S'dom. It would appear to Avraham that his plea had not saved anyone; God told him He would not destroy for the sake of ten righteous men, He obviously destroyed the cities, and nobody had said anything about evacuating <10 righteous people.
Did Avraham know that Lot survived, or did he believe, based on these events, that his nephew and his family had been killed?
This question came up in my chevruta study tonight. We didn't find any Rashi or commentaries in chumashim that address the question. I also didn't find anything in a scan of B'reishit Rabbah.