Yup, you're in the right place.
You are talking about it happening over night? Don't sweat that. Since it's such a small amount, I'd bet that it's nothing more than the instrument cooling off and settling. You probably have to re-tune after you've played a bit (10-15 min?) as well... sometime after it's warmed up from your body heat. It will probably be less after the strings are broken in too.
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