In a game, I mulliganed and put 2 cards from my hand on the bottom of my library.
Later in that game, my opponent heisted my library with the ability of Thieving Aven.
The card exiled with heist was one of the cards I put on the bottom during mulligans, and it was shown as revealed to me.
Later, my opponent heisted another card, this time not one I had mulliganed (I discussed with my opponent after the game), and this one was hidden from me, as heisted cards usually are.
The first heisted card shouldn't have been revealed to me, since I had not searched my library (or done anything else to know the positions of cards in my library), and would otherwise have no idea if that card was now gone from its previously known position on the bottom of my library.