How full the battery is, is not as trivial as it is for a normal tank to determine how much is still in it. The battery management system (BMS) of the battery can not 1:1 convert the battery voltage in the state of charge of the battery, because that would lead to errors. It has to calculate, with the help of predefined values, how mutch charge the battery still has got. The battery voltage certainly plays a role, but also things like temperature and the specific behavior of the cells.
And where did the few % go over night?
Nowhere, everything is still there. The BMS had time to re-evaluate how full the battery is right now. It may also have cooled down a bit, which does not directly result in a loss of power, but because of the now higher internal resistance, it has a little less usable power. That does not meanthat the battery is broken. This just means the BMS did its job.
If you let your electric car stand again 24 hours, then nothing "disappears".