sounds like some wire were being shorted. or else your batt wont lost most power.

sometimes if u had touched the batt term and forget to tighten up so when u draw max current to start car the term becomes open and nothing go across.
If u come back to car without power that is usually related to headlights left on, drain all the juices out.
these newer cars all seems to have a mind of her own. Few yrs ago a fnd has a contirental, he says low voltage in a batt can cause the computer to run wonky too.

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