It seems that in the latest dev patch it is possible to use night vision without actually having NVGs in your inventory. It has something to do with the binoculars.
You can cause the bug happen even if you remove the items using the init field in the editor. {F21696}
There are 2 ways to reproduce the bug
*1*
-Remove NVG's from the inventory (http://i.imgur.com/VLTPgkt.jpg)
-Night vision still works, because there are binoculars in the inventory (http://i.imgur.com/LLIhH3a.jpg)
-As soon as you remove binoculars from the inventory, night vision stops working. (http://i.imgur.com/g6gPX3o.jpg)
*2*
-Remove binoculars from the inventory (http://i.imgur.com/8hFj9GC.jpg)
-Remove NVGs from the inventory (http://i.imgur.com/w1D2ay2.jpg)
-Night vision still works, even without either NVGs or binoculars. (http://i.imgur.com/w1D2ay2.jpg)
Should be fixed in the upcoming dev build.
Thank you for helping us improve... or fix ;) the game!