Red Triangle Gen 1 Mystery

Hoping some Prius geniuses can help me get to the bottom of this mystery:

I bought a 2002 Gen 1 Prius with 200K miles last week off Facebook for 2 grand. Before I bought it, I ran a Carfax report that showed it had lived its life in the Marin/Sonoma County area from 2002-2024, where the owner had it serviced 3 times a year at the local dealership (Hansen Toyota), and kept records of these services. The car is in mint condition, perfect interior, nearly perfect exterior. It's hard to believe this is a 24 year old vehicle. I trusted the maintenance history so I bought it.

I drive it home from Fairfield,CA to San Francisco, driving it for about 1.5 hours without problem. The next morning, it was showing the feared Red Triangle of Death. I read that it can be caused by low oil and parking on an incline, so I put it on flat ground and gave it a quart. Triangle went away and ran perfectly. Later that day, I get a check engine light on for a faulty o2 sensor, which goes away on its own (I've ordered the piece and will fix soon).

The next morning, the triangle comes back on. I detached and reattached the 12v battery and the light goes away.

This morning, I go to start it (parked on flat ground) and the triangle comes back on. I reset the 12v like I did yesterday, but the light comes back after a few seconds running. I'm now at a loss for what to do.

Consider:

1. 12v battery has the number 7-26 on it, which I interpret at the manufacturing date, but I purchased the car on July 2nd, which would make it weird to have the battery made the day before. it was done at amp power batteries in Fairfield, california
   
   
2. the hybrid battery seems to be new/refurbished January of this year, but the service shop isn't on Google Maps or anywhere online. The battery service didn't appear on the carfax report. I tried calling the number on the battery's sticker (what's app only??) but it's the weekend and they didn't answer. Unsure about a warranty on it. See photos.
   
   
3. the person I bought it from sells cars professionally on Facebook and has a clean track record. He said he'd driven it around for a few months before selling it, and gave me new tires. there was no lights on when I test drove it
   
   
4. It was successfully smogged on June 22, 2026.
   
   
5. sometimes the red triangle comes on before starting (gen 1's have key ignitions) and prohibits the engine from starting. sometimes the engine starts and then the triangle comes on.
   

what could be happening/what do I do next? Is this a computer issue? A 12v issue? a hybrid battery issue? some other issue altogether?