Relatively simple to spawn the checking actors and have them report back to the tracker. It gets more complex depending on how you want the item to appear or behave.
The simplest one I can think of would be to have the secret-checkers simply make a sound when they're in a secret sector, like the pickup "boop" sound or something, so the player would be able to gauge distance and direction to the sound. Unfortunately it would be weird in multiplayer since other players could hear the sounds too, even if they don't have a secret-finder. I could make something like this in an hour or two.
Another fun idea might be like a geiger counter: The secret-finder makes noise when the player is looking toward a secret, and the noise gets louder / more frequent the closer the player is to the secret sector. A bit more difficult, but would work better in multiplayer since it could be done without other players hearing the sound.
Something very complex would be a visual radar like the motion-tracker from
Aliens. The secret-checkers could report back their information to the secret-finder and show dots on a screen or something.
