What are the "???" secret/unidentified skills?
The skills list has 17 secret/locked/unidentified skills towards its end listed as "???"/"? ? ?". They are available under certain conditions:
- Get 800 kills with grenades
- Get 1,500 kills with handguns
- Get 1,000 kills with shotguns
- Get 800 kills with Magnums
- Get 1,000 kills with sniper rifles
- Get 1,500 kills with machine pistols
- Get 1,500 kills with assault rifles
- Get 800 kills with grenade launchers
- Get 800 kills with crossbows
- Unlocked after all campaigns have been completed (x8)
What are these skills? And is there any way to track progress towards unlocking them?
As Domescoper has already mentioned, completing all campaigns unlocks the infinite ammo skills for each weapon.
The other skills that are unlocked by getting a certain number of kills with each weapon are master skills for that weapon, for example you can unlock "Handgun Master". Each of these master skills increase the damage dealt by that particular weapon.
A full list of skills including the infinite ammo and master skills can be found here.