What determines retreat path?

[Almskaar]

So I read in the manual that units can not retreat 1. into an enemy ZOC hex unless occupied by a friendly unit, 2. into or across terrain normally prohibited (i.e. a river), and/or 3. a minefield.

Aside from that if there's more than one valid hex to retreat to how is that determined? Is it random? Does it retreat toward it's HQ, nearest supply hex, hex affording the most cover, or hex costing the least amount of movement points?