I agree that the USB shouldn't drain that much. I have left things charging for days - phones, headphones, etc and not had an issue. But mine are connected through a fuse block to the battery so not the same set up.
This doesn't answer your question, and I can't explain why, but it might be helpful. I had a SEA connection on the battery for charging for years, and before this bike, it was on my other one. Then about a year ago, my battery starting discharging when parked for less than a day. Took off the SAE connection, and using the same battery, no problems since.
PS. When did the K1600 get a dash power outlet?