It was not an error on your employer's behalf: they have no obligation to inform an employee who is absent from his or her job beyond the permitted time (e.g. "beyond the protected FMLA period," as you write) that the position may be filled by another. Once you are out of work without using FMLA (or longer than FMLA permits) and/or without covering the absence by using paid time off (e.g. sick or vacation leave) you'd earned, you may  be terminated or replaced at any time, and the employer has no duty to warn you of this or give you a chance to come back to work.