Argument from silence or Argumentum a silentio

Bobby: I know where Mary lives.
Billy: Where?
Bobby: I'm not telling you!
Billy: You're just saying that because you don't know!
Billy's conclusion may not be justified: perhaps Bobby doesn't want to tell him. Consider, however, the following type of argument:
John: Do you know any Spanish?
Jack: Of course. I speak it like a native.
John: That's good, because I need to know the Spanish phrase for "Happy Birthday".
Jack: Sorry, I don't have time for that right now. Maybe tomorrow. Bye.
Afterwards, Jack continually refuses to give John the Spanish translation, either by ignoring John or by giving excuses. John then concludes, by argument from silence, that Jack does not in fact know Spanish or does not know it well. In other words, John believes that Jack's ignorance is the most plausible explanation for his silence. Use of argument from silence in this situation is reasonable given the alternatives, that Jack either doesn't want or is afraid to translate, would be unreasonable without more information.
Here is another example using the same argument but in a different context:
John: Do you know your wife's e-mail password?
Jack: Yes, I do as a matter of fact.
John: What is it?
Jack: Hey, that's none of your business.
When John repeatedly asked for the password, Jack ignores him completely. Thus, using the argument from silence, John concludes that Jack does not actually know the password. Such an argument from silence, in contrast, may be considered unreasonable, since a password is a security feature not intended to be shared with a stranger simply because they asked. It may be reasonable, by contrast, to assume that Jack does indeed know the password but refuses to say it for legitimate security concerns