I have also had random monster spawns related to quests in unreachable places.  Here is a save game with four examples:

AutoSave.EleSav

Note the two "Enemies in my territory" are stuck in a lake and on top of a mountain.  There are two not in my territory in the foothills and on the coast in unreachable/border tiles as well.

If I really wanted to I am sure I could lower the mountains to get at some of them, but this provides a good example of things spawning in unreachable places.

For reference here is the play session information that generated them:

FallenEnchantress0_86-2012-02-25T21-05-25-468.zip