This is simple Stealth system where the AI start chasing whenever the player cross his line of sight, running close to him trigger ir too as he will go after the sound, being crouched wont make any sound.
After a period of time, the AI will start searching for you around the area closely and then will expand the area of search until not finding anything or finding the player.
Chasing the player
Look at the last known position
Start searching around