Lerping doesn't return to the same position?

Well there are actually several problems I see here:

 //There is no boolean to check if we want to scope in or not (more on that later)
 public var DefaultPos : Vector3;
 var SightPos          : Vector3;
 var SightBool: boolean = false;
 var Speed    : float = 1.00;
 var AimPosX  : float = 0.00;
 var AimPosY  : float = 0.00;
 var Sight    : GameObject;
 var Gun      : GameObject;
 
 function Start(){
     //This should be the guns start position not the transforms.  Unless the transform is the gun of course
     DefaultPos = transform.localPosition;
     SightPos   = Sight.transform.localPosition;
 }
 
 function Update(){
     //This is only called when the button is pushed down so it only runs the lerp for one frame what would be better is setting a boolean to true/false and lerping over time
     if (Input.GetButtonDown("Fire2")){
       Gun.transform.localPosition = Vector3.Lerp(Gun.transform.localPosition, SightPos, Time.deltaTime * Speed);
   }
     //same as above
     if (Input.GetButtonUp("Fire2")){
       Gun.transform.localPosition = Vector3.Lerp(Gun.transform.localPosition, DefaultPos, Time.deltaTime * Speed); 
   }
 
 }

Here is a better way:

 var defaultPos : Vector3;
 var sightPos : Vector3;
 //a boolean to know if we are scoping in or out
 var scopeIntoSights : boolean = false;
 var speed : float = 1.0;
 var aimPosX : float = 0.0;
 var aimPosY : float = 0.0;
 var sight : Transform;
 var gun : Transform;
 //the percentage we are scoped in
 var scopedInPercent : float = 0.0;
 
 function Start(){
     DefaultPos = gun.localPosition;
     sightPos   = sight.localPosition;
 }
 
 function Update(){
     //Check if Fire2 was pushed down or up and set boolean accordingly
     if (Input.GetButtonDown("Fire2")) scopeIntoSights = true;
     else if (Input.GetButtonUp("Fire2")) scopeIntoSights = false;
     
     //Check if we should scope in or out, and if we have already.
     //If not done with the transition add or take away accordingly
     if(scopeIntoSights == true && !Mathf.Approximately(scopedInPercent,1.0)) scopedInPercent += Time.deltaTime * speed;
     else if(scopeIntoSights == false && !Mathf.Approximately(scopedInPercent,0.0)) scopedInPercent -= Time.deltaTime * speed;
     
     //Lerp takes a value between 0 and 1 so clamp to make sure it stays inside 0 and 1
     scopedInPercent = Mathf.Clamp(scopedInPercent,0.0,1.0);
     
     //apply position based on percent, default position and sight position
     gun.localPosition = Vector3.Lerp(defaultPos, sightPos, scopedInPercent);
 }

Hope that works out for you.