They knew exactly where they were....they had only been lost to scripture. The first century historian, Josephus, mentions in his "Antiquities" Book XI, Chapter V, Paragraph 2 that there were only two tribes left in Asia (minor) subject to the Romans, the other ten were beyond the Euphrates and were beyond counting.....they were so many!
Also please see this.... Babylonian Jewry
Don't you think that divinely appointed Apostles would be able to find these folks.....if they had been told to do so?
Well he sent out the 12 and they couldn't have been gone for long as they were back for other events in His life. Plus they went on foot. The tribes were assimilated into other cultures by then, in fact, were probably considered Gentiles. Even today, there is speculation on who they are, and some evidence to support it, that they dispersed just about everywhere.
Do you ever read "Debka.com"?
Didn't I read that Babylon was just a code name for Rome??? But then what do we call Babylon???