Sleeper can be kept invisible, it depends on how the deck is handled. If you want to square the deck: keep the gaff on the bottom of the deck. Retain the gaff on the bottom of the deck when shuffling, ribbon spreading, fanning etc.
When the gaff needs to go into the middle of the deck, shuffle it there, or multiple cut it there. But do not fully square the deck. Keep it loose. Dribbling the deck is another option.
The small steps caused by cutting or shuffling, hide the gaff. This is true for other gaffs, not just Sleeper.
Shuffling, cutting and keeping the deck in a loose condition adds to the overall impression that it's just a deck of cards.
Hope that helps!