Usually Document CALs get lost because you delete a document that had Doc CALs attached without first releasing them. It's not really true that they are lost. You don't have any access to them anymore due to the fact that the Document CAL panel is attached to the original Document. BTW the same logic applies to reload tasks attached to deleted documents.
In the future, before deleting or renaming a published document, first release all Document CALs and delete any reload tasks/jobs.
If you don't have a Publisher, check whether there are documents with an all-captials name in the tree root in the QMC Documents tab. If you have a Publisher, also look for "Orphans" folders that have content. Deleted documents that still had CALs or tasks attached will reappear as ghosts. Create an empty QVW (0 bytes length is ok), name it after the deleted document and put it in the same folder as the original document. The Reload & Document CAL tabs will reappear. IF you cannot find any all-capitals document names in QMC, use the Server CAL Manager to recover really-lost CALs, as Petter suggested.