Well~I am not OP, I am just pointing out that the stationary gun have to be destroyed.  Based on my experience that every time when there is a stationary gun within proximity of the target location, it has to be destroyed in order to move on.  I was here trying to tell OP that he missed out the gun, I didn't know that wasn't intended.

 I don't find it a "problem" since I know it is considered as an enemy and destroying it is fine for me, however, some players may be confused just like OP.

Not sure if that is a enemy turrent, I mean those that need to be manned and can be used by the player.