Making objects visible when nothings collide with it

I use these codes to make “ArrowObj” become visible when another game object collide with “HelpStartBox”, it works:

public var ArrowObj:GameObject;

function Start () {
	ArrowObj.renderer.enabled = false;  
}

function OnTriggerEnter(col:Collider) {

	if(col.gameObject.name=="HelpStartBox"){
		ArrowObj.renderer.enabled = true;
		yield WaitForSeconds(2);
		Application.LoadLevel (1);
		Debug.Log("1");
		}
}

But when I want to make “ArrowObj” become invisible when nothings collide with “HelpStartBox”, I don’t know what to do. I have tried to put these codes but it didn’t work.

else {
	ArrowObj.renderer.enabled = false;
}

What should I do?

do function OnTriggerExit() and put ArrowObj.renderer.enabled = false; that bad boy in there