Actually this does raise a good point because when Michael tells Appollonia's father that he is an American hiding in Sicily, he warns him not to tell this to anyone lest Appollonia lose a father instead of gaining a husband. Next thing you know he is traipsing all over town, being watched by gossipy old women and then having a huge public wedding. It just goes to show how irrational people become when they fall in love.
As for Fredo being smarter about hiding, I don't think so...he was out of sight because he spent his days and nights banging cocktail waitresses two at a time, so players couldn't get a drink at the tables.