Triangle of Death - Appeared then Gone

Hi all, first time posting but have just read a bit.

Background: I got a used 2006 Prius a few months ago. The battery in it had died (before I bought it) and was replaced with a newer battery from a totaled car. The new battery was working perfectly fine, the whole car was running great, mechanic (specializes in foreign and electric cars) ran tests and everything was working fine! He had also replaced part of the heating/cooling system (I believe the heat wasn’t working).

The other day I drove up my (very bumpy) country “driveway”. As I pulled out the red triangle and I believe a brake light(?) came on. I pulled over nearby and turned the car off to restart. It wouldn’t restart, the power button was red, all the dash lights came on, and it would only go into neutral. Someone pulled over to help. I read on here about the battery safety plug. He pulled that out and reinserted it. Still didn’t work. He went and got his reader and the code POA0D and P0AA6 came up. Tried clearing the codes. Didn’t work. Towed it to the mechanic. 4 days later (today) the mechanic resets the safety plug and it starts, no codes. He can’t see what any of the original sub codes what have been because they were cleared.

Now, he’s not confident in this was the fix. He’s thinking if the triangle came on in the first place something is wrong. I’m wondering if anyone’s had a similar experience - could the triangle have been a fluke? Is there still something wrong? Can it go on then off then on again?

thanks all! I’m stressed and don’t know what to do! I just got it and was so happy with it..