Visual and aural player detection
A short, but very informative video by David Rosen on one way to approach player detection in a stealth game.
Check it out here:
The video is actually part of the devlog for David's upcoming game Overgrowth and his devlog contains many equally interesting videos.
David has also made some games in Unity for game jams, two of which are available with complete source code from his github. They are Desperate Gods, a digital board game, and and early version of Receiver (available on Steam). Get them here:
They are great projects for learning from, but remember to read the license for each project if you plan on doing anything else with them.