Battery drains while car is sitting

The battery sometimes drains after sitting for a few days/weeks (most recently 2 weeks). Only the dash lights come on and you hear the clicking sound. The original battery has been replaced and now, after about 2,000 miles on the new battery, and driving the car a few blocks or miles every 2-3 weeks, it's started doing it again. Can't find anything that's obviously on when I search inside the vehicle.

Transit Connect