Is it a Volkswagen?

A friend of mine had the same problem with a new-ish Golf. He went to the dealer and they said it was "normal", and tried to sell him on plug-in thingy to keep the battery charged. Not particularly useful information, but there it is.

One thing I would check is if there's any single device draining power. Sometimes just leaving a cell phone charge cable plugged in can do it, or maybe an aftermarket stereo.