Probably related to the fact that one can operate unpurchased turrets at the end of upgrading time as I described [url=https://www.bungie.net/en/Forums/Post/264476202?sort=0&page=0&path=1]here[/url]. Sometimes for whatever reason after upgrading time runs out unpurchased Operable Turrets do not disappear and remains on the battlefield. They can be operated and behaves as level 1 Operable Turrets. However, after the next upgrading time in the same area the unpurchased Operable Turrets would properly vanish. In the attached clip I find an unpurchased Operable Turret in a different area. Later in the clip, after upgrading time has run out that turret is still present. I joined that run in progress, so I assume the turret appeared after 7th wave (or 17th, 27th, 37th) and then ADU never appeared in its area.