Batteries can do weird things sometimes. They can have internal shorts that slowly or quickly drain them. A voltmeter is also not a good test on a battery, you need to do it with a load tester, and even those are not 100 percent sometimes.

We chased an electrical problem on my ex-GF car and finally found that the 2 year old battery was not making the grade any more. In Florida, with our high heat all the time , batteries don't last very long. When I was in the marine business I recommended people should change them out at the 2 year mark because they will let you down when you least expect it, and tows on the water are very expensive...........like $ 1000 expensive. So changing the battery was good insurance.

Don