12v battery died for no reason

So I had an appointmen to bring my car to a shop to repair a damage from an accident. Long story short, the car had no battery problems prior, drove to the shop 22km away, shut it off, went inside to hand over the keys, forgot something in the car, went back to get the keys not even 5 minutes after shutting the car off, and the car won't respond to the unlock/unarming. Luckily I didn't lock/arm the car, so I was able to get inside. Car wouldn't start.

Fast forward about a week, the car is fixed. The tech at the shop had to keep jump starting the car to move it. But when I went to pick it up, they couldn't jump it to move it anymore. Red triangle came on, "problem" message came up, etc. etc.I can get the car into ready, but whenever I shift to R or D, it dies. They kept the booster/generator thing on the car for about 15 min. and they gave up because they had to close the shop and go home and said I can stay there and wait for a tow truck.

Anyways, BCAA (a roadside assistance service) sent a service guy over around 2 hours after the shop closed and was able to jump/boost the car. Car started, no triangle, and it was ready to go. While I was waiting for BCAA, I bought a new battery from a shop nearby.

Drove the car home, parked, shut it off and bam, dead again. Too tired to worry about it and went to bed. Next day, woke up, changed the old battery to the new battery. Took the battery to an outlet and had a charger pack charged it for about 8 hours, got it to full according to the charger pack.

So my question is, how can a battery just suddenly die like that without any warning? It's the original battery. Is it a bad now? I'm really hoping to god it's not anything to do with my car, like a parasitic drain or something and that the battery is in fact bad. How can I test it? It's sitting outside my door right now, fully charged.