Is it a bad ground?

I have a 2000 mustang GT (hers) and last few
months it randomly doesn't start. Alternator
charges, battery good, starter is on point always.
She may have to jump it twice in a day, it can sit 3
days and bust off with no hesitation. Car always
makes her look bad cause never shows issue when
I drive it. 2 times now it has, both times I pulled up
to get gas, cut it off. No music or anything, pump
gas turn key and nothing... No clicks, no starter, no
lights , nothing! Hook up cables and instantly
cranks. May do it next day, may be a week later..
sensors and computers not my thing, always
worked on older stuff.. could it be grounding out??
Where do I start?