Enemy looks for you, when he can't see you

Okay I have a script when you in the line of sight he will follow you, I am also using Nav Mesh.

What is want to do is, if the Enemy cannot see the player anymore I want it to look around for a certain amount of time then go back to its starting point.

 var walkSpeed = 3;
 var rotationSpeed : float = 3;
 var runningSpeed : float = 6;
 
 var player : GameObject;
 var myTransform : Transform;
 var target : Transform;
 var fieldOfView : float = 50;
 var rayRange : float = 10;
 
 var distanceToStopRunning : float = 10;
 var distanceToStopMoving : float = 3;
 var detectFromBehind : float = 2;
 
 var sneakingBehind = false;
 var chasing = false;
 
 function Awake(){
     myTransform = transform;
 }
 
 function Update(){
 
     var hit : RaycastHit;
     var rayDirection = player.transform.position - transform.position;
     var distanceToPlayer = Vector3.Distance(transform.position, player.transform.position);
     
     //Checks if to see if player is infront
     if((Vector3.Angle(rayDirection, transform.forward)) < fieldOfView){
         if(Physics.Raycast (transform.position, rayDirection, hit, rayRange)){
             if(hit.transform.tag == "Player"){
                 Debug.Log("Can see you MF");
                 lookingAt = true;
             }
         }
     }
     
     if(lookingAt == true){
         chasing = true;
     }      
   
         
     if(chasing == true){
         GetComponent(NavMeshAgent).destination = target.position;
         if(lookingAt == false){
             
             
             
         }
     }
 }
 
 function OnDrawGizmosSelected(){
     Gizmos.color = Color.green;
     Gizmos.DrawRay (transform.position, transform.forward * rayRange);
 }

Thanks for the help.