Curiosity gap
Also said as Information gap, Mystery box
The itch of knowing that there is something you do not know. George Loewenstein described curiosity in 1994 as a gap between what you know and what you want to know.
It only works if the player already knows a bit: with nothing to go on there is no question, and with everything there is no reason. It is cheap to build and it does not wear out as rewards do. The trap is a promise you cannot keep, since players remember a mystery that led nowhere.
Say it
“Put a locked door with a light under it in the first room, in the player's line of sight. Do not mark it on the map. Let them find the key an hour later.”
Where you’ve seen it
Outer Wilds runs almost entirely on this. Nothing you gain makes your ship stronger, only what you know changes, and each answer raises a new question.
Not to be confused with clickbait
Both open a gap. A curiosity gap pays it off with something worth knowing. Clickbait opens it just to collect the click.