The other object that was never mentioned, only implied
At some point during the series, Joe must have come inadvertently into possession of an Object that gives him extraordinary powers of persuasion.
That's the only explanation for his ability to talk various characters into handing over valuable items to him simply by asking nicely.
"My 40-year-old mystery photo of me and my non-husband? Sure, take it."
"You barge into my office and 30 seconds after I order you killed, I'm showing you all the contents of my secret database. It just suddenly seemed like the right thing to do.
"You want to borrow my car and drive it into the desert? It's all I have! But, okay."
"You want to borrow my magic bus pass? Sure, why not?"