I did some experiments with enemies near map border and it seems like if an
enemy once touched a map border (with no level next to iz) it was still walking
into some direction beyond the map, but instead of crashing the game with 
out-of-map error, the game kept the enemy on the current level.

The enemy was continuously "walking" away but not moving.

 r5036

What is the expected behavior?
Why should we care about levels that open to a void? They're not a realistic case.