Hi,
I have both version of XPS 13 9350 and 9360.
I fully charge the battery to 100% and then shutdown the computers and turn it off.
If I start them again after 2 days I have lost ca 10% of battery charge - only 90% charge left, although they were completely turned off (no standby!) ....
After a week without turning on, I lose 20-25% of charge. As written without working on the XPS 13.
I also have a Dell Latitude 7280 from work and it loses only 4% charge within a week. This seems normal in my opinion.
The battery was exchanged for the 9350 by Dell but this have brought nothing. Something drains battery on the two XPS when switched off.
Someone have an idea or can even report how it is with you?