For background, I bought my Prius used in the early 2015 with about 160,000 on it.
After a few months I noticed that the hybrid battery would be systematically drained or nearly so upon starting the car, even if at the moment of turn it off it had been full or nearly so. The car struggled going uphill. I found the 12v battery drained one day (yes, will all the flashy lights on the dashboard, then nothing, etc). I replaced it and I found the new one drained a few days later. (Note that I do not drive my car that much and sometimes do not use it for more than a week. I have a motorcycle.) I still have that same one, though. I just charged it up and made sure I drove the car at least once a week. But the other issues remained. I though the hybrid battery was going bad and was pulling on the 12v battery somehow.
What do you think was happening?