The turret in the random events, rarely happen, but if this event appear in a Purge mission, it would consider as one of the enemy need to be killed to finished the mission, the issue is, if u are to quick kill the enginners building it, it would stop building but still a enemy need to be killed to end the mission, but you cant kill it cuz it was not build up so no hit model for it, and the mission could not be finished by any chance.