The NSV is occupied by a bot that didn't spawn yet.
This can happen in any emplacement found in the game world.

It seems that in these cases the inscription RESERVED should be written and the action should be blocked

better if the unspawned bot leave from the mg if they haven't materialized in the world, so a player can use it. Or better yet, have the bots occupying machineguns be prioritized for spawning into the world so they can actually do some damage on servers with more than 20 players* doing their own thing. Right now, if the server is at least moderately populated, you can run a logi truck or roam around without really fearing the prepaced FIA machineguns since they are unlikely to spawn.

* AIs only spawn near players, so the more players there are, and the more spread out they are, the more likely you are to run into this issue of AI teams not spawning in.

The problem isn't with this particular machine gun. I had the same problem with another one.