Then it struck me that later in the novel, a villainess will use his being alive to threaten the people who murdered him. (She's kind of annoyed that they failed.) And other villains may mention him, on account of he's willing to make life difficult for them.
BUT -- I never put in where they learn it!
Gotta decide how many of the villains learn it. And it may take more than one means given their degree of trust and communication. . . .