Does a stealth event allows partisans to exit the map?
Just because 7.2 2nd par states "....(as if the dark gray border were another hex)....
No. R15 states that ...he removes that unit/marker from the hex that it occupies and places it into an adjacent hex. The map edge is not an adjacent hex.
Further 7.2.1 (Voluntary Exit) only mentions Advancing or Moving as means of voluntarily exiting the map.