The only thing I have found so far, about leaving the table of battle comes from the section about “All Alone” checks, which says the Character is removed from Combat. It doesn’t say the Character is OOA, meaning wounded. There are some Scenario specifics stating about carrying an item off the table. I haven’t found anything yet, about Volunteering to run off the table, nor anything that stating all models must remain until End of Game.
My Group plays it that a model can run off the table and runs back to camp. Once the Character leaves the table, He/She can not return back for that current game. For instance in the Scenario Wyrdstone Hunt, when a Shard is picked up by a weak fighter, its not uncommon for that fighter to run off the nearest table edge or back through friendly warriors to the table edge; which ever is the safer route. Warriors that have run off the table count towards Route Tests as the Warband’s numbers on the table lower.