Capcha fails without specific scenario

I had trouble getting the capcha widget to validate. but when I added "on"=>"contact" it works. Is this the intended behavior? I saw no documentation detailing that I needed to specify the scenario.

Could you post your captcha validation rules that you used?

		// NOTE: you should only define rules for those attributes that

		// will receive user inputs.

		return array(

			array('lead_name', 'required'),

			array('lead_name', 'length', 'max'=>64),

			array('lead_company', 'length', 'max'=>128),

			array('lead_email', 'length', 'max'=>256),

			array('lead_phone', 'length', 'max'=>16),

			array('lead_description', 'length', 'max'=>3000),

			array('lead_title', 'length', 'max'=>56),

			// The following rule is used by search().

			// Please remove those attributes that should not be searched.

			array('lead_id, lead_name, lead_company, lead_email, lead_phone, lead_description, lead_title', 'safe', 'on'=>'search'),

                     // verifyCode needs to be entered correctly

			array('verifyCode', 'captcha', 'allowEmpty'=>!CCaptcha::checkRequirements(),'message'=>'captcha fail!','on'=>'contact'),