Peeking behind the curtain

[i_confusion]

Once a plot has been completed in an RPG game should a GM divulge information that the characters wouldn't have know and are unlikely to find out about? I feel like I have a terribly big secret and I'm desperate to spill the beans.
It's bad in my current game as the NPCs involved may reoccur.....but I have previous game whereby we MIGHT go back to it. Oooh. It's sooo tempting.