My vote is corrosion between a post and battery clamp. Had it happen numerous times to me over the years. And it's crazy how the slightest, sometimes imperceptible amount of corrosion, can flow plenty of current - until - you want to start it.
Easy to diagnose if you have a DVM and a second person. Pick a suspect battery post (you have only 2 choices!), put one lead on the middle of the actual post and the other lead on the clamp itself. When you engage the starter, a problematic connection will show multiple volts of VD.
This is a common problem in my wife's 02 Sienna. I have a small hammer in the car for her and she knows how to slightly "tap" a battery clamp from the side to rotate it just the slightest amount. The car starts, she let's me know, and when I can I take the connection apart and wire brush it.
And now a 1 minute story that is on topic. I know almost everyone here likely had a person in their younger life that really showed them how things worked. Mine was Bill Burns, a wonderful Irish gentleman whose day job was working for the MTA (Subway) in NYC. Bill was unafraid to tackle just about any kind of repair and his "been there- seen that" list was quite long! One day in Brooklyn, late 60s or early 70s the family '65 Impala station wagon won't start. Bill comes over, no tools or meters, pops the hood, turns on the headlights, tries to start the car, goes hmm, then asks me to run to the basement and get a hammer and real small finishing nail. Both puzzled and curious, I do exactly that while wondering how he's going to fix anything with a hammer?! And a nail ?!?! He takes the small brad and hammers it down between the outside of a battery lead post and the inside of the clamp. The car starts right up and he explains what the problem was, what his quick fix did and that it would be perfectly fine for a few days until we could take it apart and clean it properly.
