If an enemy unit has no valid path to get to an adventurer, it will move as far as it is able toward the closest opposing unit.

This is supported by the following from page 65:

• "The destination hex must be the closest hex where at least 1 targetable unit would be in range. If that is not yet possible, the enemy will move as far as it is able toward the closest opposing unit."

Therefore, even if the enemy cannot reach a hex where an adventurer is in range, it will still move toward the closest opposing unit along the shortest route possible, as far as its movement allows.

Source: page 65.

Source: Core Rulebook (pp. 58, 65, 67, 89, 90)