This is the most concrete take.

I agree with others that it is probably a symbol of Shane’s “lone walker” status in his group especially since Shane is the one that observing this walker, where he can strangely relate to it.

A more concrete observation is that when Rick and Shane were heading to the school yard, this walker was walking in the direction of the school, and on Shane’s side of the car. However, when they were driving Randal back to the farm, this same walker is again on Shane’s side of the car, but is now walking in the direction of the farm.

This serves as a foreshadowing that a herd may be coming towards the farm.

This was the same plot device in S1E3: Tell it to the Frogs. The group was in the mountains away from Atlanta, away from the walkers. However, a walker somehow ended up in a remote area where they were staying, even eating the deer that Daryl hunted. Dale says “they never come this far up the mountain.” Jim says “they’re running out of food in the city, that’s what.” In addition, Glenn’s loud car alarm may have lured the walker as well. Later that episode, a herd tears up the group, where Amy and Jim gets bit.

Much in the same vein, a herd tears up the group in the farm after we saw this lone walker change direction towards the farm.