If you are retreating from combat would that not be to the previous tile which should be hero free?
That's an excellent point. Maybe that's why the book doesn't address this directly. Indeed, page 12 of the rules (3E [edit: 3E Revised]) say:
When a Hero successfully escapes, he must leave the Dungeon chamber where he encountered the Monster and return to the Dungeon chamber he entered from.
So... problem solved. Colin for the win!