We need a pure language phrase for "Easter Egg"

This game contains at least one sly reference to one of Eric’s books, All Good Things. Traditionally references like this in a game are called "Easter eggs", but we need to come up with a better, more Witness-friendly phrase to use.

It should preferably be a phrase that references a hidden thing that can be found if searched for, and it should be a biblical or theocratic term. Like, a "hidden treasure" (Job 3:21) or a "pearl" as in the pearl of high value (Matt 13:46), or "teraphim" like the idols Rebekah hid from her father (Genesis 31:34).

Ok, maybe not that last one...