Since the last Hotfix Recon Darts sometimes double-fire. When you pull the trigger, instead of firing one dart, 2 come out of the gun at a slight offset to each other. If this would put you under 0 ammo, it 'steals' one of the darts you already have out and re-places it.
For example, I have 3 darts. I fire one at a wall near me, one at a building to my left and one at a building to my right. The double-fire glitches on the 3rd dart, making the dart on the wall near me disappear a reappear right next to the dart I fired at the building on my right.
Oddly, if you would have enough ammo for the 'extra' dart, it does not actually take the ammo.
For example, I have 3 darts. I fire one at a wall near me, one at a building to my left and one at a building to my right. The double-fire glitches on the 1st dart, making the dart on the wall near me clone itself into 2. This takes no additional ammo, and I end up with 2 darts on the wall near me, and 1 on the building to my left and 1 on the building to my right, for a total of 4 darts.
Replication? Fire a bunch of darts.
Link to Bug Thread on PS Official Forums. Please Upvote!