In my honest opinion, Goblins especialy should know well enough wich travelers is safe to ambush ... and wich would be best to leave alone. Even if they would learn by trial and error method.
I gotta say I disagree with this. If they did, I probably wouldn't have killed as many of them in my adventuring days (ahem, always in self defense, I assure you). Just saying they tend to think themselves a MUCH bigger threat than they end up being.