What should be done if a document is lost within the organisation when there is certainty (100%) that it did not leave the company and was probably only attached to other documentation?

ANSWER

Such an event may be classified as a security incident, as the effect of accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or unauthorised access to personal data contained in the document did not arise (provided that as a result of the attachment described, the document did not reach persons within the organisation who are not authorised to process the personal data contained in it).