Why is it I see deer around my property all season long, but a week to 2 weeks before season starts it seems like the deer just disappear. Like they know its hunting season. Are they that smart?

Deer change their patterns and behaviors several times throughout the year - primarily based on their preferred food source.  Throughout the summer, deer spend a lot of time in agricultural fields - soybeans, alfalfa, etc.  They are pretty laid back and easy to see during daylight.  As fall approaches and acorns start to fall, the deer tend to move off the fields and into the timber, which makes them more difficult to see.  They are still there, you just don't see them as much.  Once hunting season starts and there is more pressure in the woods, deer will begin to move more during night and less during the day, making them seem like they are gone.  Have patience - once the rut gets closer they will start moving during the daylight a lot more often.