Without stepping on Coop's toes because this is a COMICS spoiler
In the Comics, there is a group of people that didn't meet the requirements to live in the safe zone. They could be the Wolves.
There was graffiti about 3 episodes back about 'The Wolves' and this could be that same group.
Another theory that stems from the comics are the "W" stands for 'Whisperers'. People that live among the walkers and wear their skin and this keeps them from being attacked.
I don't think this one is true. The blonde tied up to the tree and fed to the walkers already had a W carved in her head. I think she was marked by the 'Wolves'.
All guess work. But something is going to happen next week and we'll know!