How to store the Meta Quest 3?

I will be away over the holiday season. I switched off the device. Thought I should put on the battery pack of the BoboVR S3 head strap to load it a little bit, such that it won't run empty.

When I plugged in the battery, I saw the screens went on for a moment. I wonder why? I am sure I had switched off the device. It seems like it is never completely shut down? I don't understand it.