BA ATW
I have LITERALLY seen this..much to my chagrin. On the way back from SIN, CSD stood in the doorway greeting, but as soon as the aircraft was in the air, she came into the F cabin, took off her shoes, wrapped herself in a Blanket, fully uniformed and curled up in 5A! Remained there until lights out, then put out the bed, and got under the duvet. Stayed there until seat belt sign for landing came on, then got up and strapped into crew seat!
Can I seriously suggest that in such a case you have a quiet word with one of the pilots on board. I can assure you that she would not remain there for long. Don't worry about be found out as the person who dobbed them in, a quiet word on the interphone will have the desired effect with no recriminations. The CSD by any such action has broken several rules and will be bricking it very quickly and likely be on a disciplinary.