I bet that a confident scammer is prepared to deal with things like that. They want to put you in a state where you are under time and emotional pressure and your "relative" will have a well practiced response why they can't answer your weird questions.

Imagine your crying grandson who caused a traffic accident in Mexico and the police planted drugs in his car and now he needs money to pay them off. He is in pain and probably has a concussion (explanation why he can't remember what you are asking), the police is hassling him to get off the phone (time pressure, explanation why the quality of the call is terrible). Will you get hung up on some code word he asked you to memorise years ago and you can't even know where it is anymore? And if you bring it up he just starts crying and tells you that you are his last chance to turn his life around. And you remember when he was a wee little kid and he fell and scraped his knee and you comforted him. Just the thought of pressing him on the code makes you feel like a terrible person. Or not. And then the scammer just finds someone more gullible. Theirs is a number game after all.