Bill K hit the nail on the head. I see more squirrels when I'm deer hunting than at any other time.
During my deer hunts I've observed the squirrels and found that they are very active in the morning after sunrise and in the mid afternoon before sunset. I don't stalk them but have had great succes finding a good spot and sitting and waiting at the base of a tree. It's funny how they keep showing up no matter how many of their buddies you've already blasted.
Squirrels tend to see you before you see them when you're walking through the woods and tend to circle around the opposite side of a tree as you walk past and you never even knew they were there. That's why I prefer to sit and wait.