I mean, can we really say that William "disappeared"? He literally just skipped town. I think it's more like Helen and Leia were too hurt by him leaving to go look for him than they not being able to find him. I mean, when Leia needed to reach him, it literally took a single phone call lol
that's what we like to call "Ocean Logic"
another example : it's pretty late at night and it's raining ,Leia is in her car with the cellphone on her hand , what does Leia do?
she gets out of the car to make the phonecall under the rain ! ............ "Ocean Logic"
look don't try to argue about the story or script on Ocean's avn because the guy barely know how write a christmas card .
just imagine how Ocean looks like everytime he grabs a pen !