In one of the first episodes, after arriving on the farm, someone made a big deal about asking whether or not they had closed the gate behind them. I think it was when they were rushing Carl into the house after being shot, not sure though.
So I assumed the farms perimeter was fenced in, and that's why they felt safe in the tents.
Yes I just watched that episode. Maggie asks Tbone and Glenn when they first arrive. Glenn and tbone are asking each other do they just walk in or what and then Maggie speaks and she is sitting on the porch and asks if they closed the gate.