Nissan,Battery keeps dying for no apparent reason

The battery on my car keeps dying out. I had it replaced several times. The last one I installed was an expensive Nissan battery. The dealership checked the battery and the car. Everything checks out fine. But, the battery keeps dying out if it sits for more than a couple of days. It was jump-started several times. There is obviously something that keeps draining the battery, but they don't know what it is. Do you have any suggestions? Thank you in advance.