Even when parked connected to a charger, the battery pack will lose a little bit of charge, and periodically start charging again to get back up to the desired level. This is normal - and a good thing to do, since it's likely better for the battery to slowly discharge a little and periodically recharge - than to keep it at a constant charge state.
When parked, not connected to a charger, there will be some charge used to maintain operations of the vehicle - and again this is normal. If parked outside, subject to high and low temperatures, the climate system will run periodically (especially if Overheat Protection is enabled), to keep the battery pack within its operating temperature range, and to avoid having the cabin interior get too hot.
There are ways to reduce battery loss:
- Set the vehicle to "Range Mode" - which will reduce energy consumption while driving. It puts the climate system in a more efficient operating mode, and also more efficiently uses the dual motors.
- Disable Smart Preconditioning and Overheat Protection, which automatically turn on the climate system.
- Park the vehicle under cover or in a garage, out of the direct sun - which will avoid the high heat caused by direct sunlight.
- For MCU1 vehicles, in the display settings, enable the energy savings mode, and uncheck "Always Connected". This allows the onboard processors to go into a deeper sleep mode, reducing energy consumption while parked. This will require a few extra seconds when re-entering the car, to allow the onboard processors to "resume from sleep". These settings do not appear to be available on MCU2 vehicles, evidently because the MCU2 design has improved energy usage - and no longer needs these settings.
- For long-term parking, you may also save some energy by turning off the climate systems when leaving the vehicle, though it's not clear doing this has any actual impact.
When parked outside in the heat and cold - you could see energy loss for 10-15 miles per day (or more), depending upon the temperature of the battery pack and interior (and if you haven't enabled any energy savings features).
For long term parking, inside a garage, under cover, in temperatures around the battery pack's operating range (60-90 F), you should be able to reduce energy consumption down to 1-3 miles of lost range per day, when all of the energy savings settings are enabled. That's what we experienced recently when our X was parked at an airport garage for 12 days.
Though... If you are only driving short distances each day, and charging overnight - don't worry about it - keep things simple, and keep the vehicle comfortable for when you return to drive...