Where the unauthorised absence of a patient who is subject to a short-term detention certificate or a certificate under section 114(2) or 115(2) of this Act authorising continued detention ceases within the period of 13 days ending with the day on which the certificate would, but for this section, have ceased to authorise the measures specified in it, the certificate shall continue to authorise those measures until the end of the period of 14 days beginning with the day when the patient’s unauthorised absence ceased.