I had the same exact problem with my 04 D600. The dealer told me, basically, that the bike just freaks out when it's not ridden. The old battery I had in my bike couldn't even hold a charge no matter what I did. I had it hooked up to a jump box and didn't even get lights! I recommend taking your battery to the dealer and seeing if they can figure it out or just get a new battery. After I dropped a new one in it started right up.

I also invested in a battery tender, just in case.