Sometimes they just hole up, plain, and simple.
When I hit a new place, and it seems to be void of squirrels, I then start scouting very closely for sign. The easiest of course is the nests, but even more so, I look for trees where they've been feeding. If you're finding any sign, I'd come back like the others said, and try again. I've hit a new place, and seen nothing quite a few times only to come back a few more times, and realize it's one of my best hunting spots.