I have just loaded up an end-of-game save and I don't have a diary entry for ZEP  (or ) either. I do have  and , though. This may be deliberate, as the later ZEPs stay in your inventory and can be read from there.
The next ZEPs should spawn when you pick up , so you could try reloading an earlier game and picking up  again. If that doesn't work, you can spawn them in manually using the ESC-F4 menu. They are zep_doc_26, zep_doc_27 and zep_doc_28. The documents should appear on the ground near you.
You are pretty much at the end of the game as it stands now, and there isn't much more you can do after finding these documents. We are awaiting a new patch that will complete the main story, but it is still being tested and I don't know when it will arrive. However, TecnoBacon has warned us that it will require starting a new game.