In the 1.30 source, I can't find any instances where game.map.enemies is being removed from or cleared, but I do see objects being added to it. I figure it's a possible contributor to the game's gradual slowdown (like a memory leak, I guess), because even though the objects in the list are being deleted, the list still grows. (this is because objects are replaced by null when deleted from a list)
game.map.heros is removed from when heroes die, so that one's not an issue. There may be others that do this, though.
Lummox JR provided a simple way to clear all nulls from a list in a forum post I made a while ago regarding this "replace with null" behavior:
// the semicolon makes it one statement
// which removes all nulls while there are
// nulls to be removed