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[Star Wars] How did Director Krennic find Galen Erso on Lah'mu?

What would attract Krennic if Erso wasn't emitting anything into space that would identify him or talking to anyone else?
gpt
Time, money, and inside knowledge. Krennic had been friends with Galen for years and knew him better than almost anyone else. Erso was a brilliant man, and farming really was quite below his talents. It's quite possible that when they got supplies, he also would request reading material and the like to entertain him and keep his mind fresh. Krennic could have set up some sort of alert any time certain code words or topics were accessed from imperial sources, and looked for a pattern. It would be a little unusual if a certain library on a remote planet a few hops away from Lah'Mu kept downloading recent papers on high-energy physics or what not. Given enough time and resources, as well as scouts, they could have tracked him down.

And there were ways to narrow down the search. Erso would almost definitely avoid any larger populated planet or planet with a large imperial presence where he could be spotted, or planets with a large criminal contingent who would claim an inevitable bounty. Would also need to be habitable for humans, and not an overly harsh world since they were raising a young daughter. Still a lot of galaxy to look at, but that would likely narrow it down to planets similar to Lah'Mu. Outer rim planets with small populations, habitable to humans and with not much else. 

He could also comb that with any possible details
he could glean from their escape. They escaped from Coruscant, and had accomplices who might be less hidden. Work that angle and try to get them to talk or to get them to at least slip up and give him a clue. Surveillance of any and all known associates. 

The fact that Saw was able to respond to a message in just a day or so at most suggests he was at least somewhat local, and if he was known as a associate of Erso, would really help narrow things down if he was in the general area.

It still took him four years.