It's not that that big of a leap. It's heavily implied Marion got stuck in Nepal after Abner died, when she didn't have enough money for passage home. So all the Nazis have to do is go to Abner's last known destination and start poking around for Americans. Of course, Indy didn't know where he was either, they hadn't spoken in 10 yearsr. He mentions rumors about Asia, and then he tells Marcus he thinks he knows where to start looking. Either his first guess was right or he did some sleuthing during the travel montage. She's not hiding, they will find her eventually.