From rule 5.0:
Units not marked as Used, that start in the same hex as an enemy unit may move out of it if otherwise able, but must first pass a Prof Check to do so.
Units that start what? Start the turn, or start their movement?
An example: at the start of my turn, one of my infantries does not share the hex with any enemies. Later on, when I want to activate it, it is sharing the hex with an enemy. Can it try to leave the hex (because the enemy was there at the start of the movement) or not (because at the start of my turn the enemy was not there and now it is)?