A cause of action against a public employee or former public employee is not barred by Section 950.2 if the plaintiff pleads and proves that he did not know or have reason to know, within the period for the presentation of a claim to the employing public entity as a condition to maintaining an action for such injury against the employing public entity, as that period is prescribed by Section 911.2 or by such other claims procedure as may be applicable, that the injury was caused by an act or omission of the public entity or by an act or omission of an employee of the public entity in the scope of his employment as a public employee.