Some kind of invisibility hack.

[Cassp 2]

Two players named 'Northern bandit lance' and 'Northern bandit Justin' on Seattle 8 Were using some kind of invisibility hack and were also speaking to each other on global and group channels on a server with neither of those unlocked.

I can't exactly prove that they were invisible, but we could both hear them speaking on global chat about us as they killed us, one even said "He just stood there as i killed him". and also started playing trollish music over global chat, so it seems these aren't out to make friends anyway.

Both myself and my friend died in spots where it is impossible to get shot without seeing the shooter.

[Ganjastar 53]

they are using older versions of the mod files. its not a hack, its an exploit.