In general, "discard" without other specification always means from your hand, in the same way "gain" means to your discard pile.
This is wrong. Tunnel uses the word "discard" without any locational specification, but it means from anywhere. Same wording as Beggar.
Okay, and Inn uses the word "gain" without any locational specification, but it means from anywhere as well. (That is to say, if you gain Inn from the trash via Graverobber, or from the Black Market deck, you still shuffle Action cards into your deck.)
So to specify: an
instruction to discard something without other specification means from your hand, in the same way an instruction to gain something means to your discard pile. "When you gain" and "when you discard" don't have those interpretations.