Master warning light after hybrid battery replacement

I just bought an 07 Prius off craigslist, my second Prius, and it drives great except that I have to disconnect and reconnect the 12v battery every time I want to start the car. About ten seconds after starting the car the big red triangle shows up and stays until I turn the car back off (particularly annoying because it won't let me use cruise control while the master warning light is there.) The guy I bought it off of says this is all because it recently had its hybrid battery replaced, and that after driving it 300 miles the system will reset or something and the warning light will go away. I am inclined to believe that the hybrid battery really was replaced because the mpg is great just like it was new and not 17 years old, but I'm not sure about the 300 miles thing (I just bought the thing yesterday so I'm not there yet.) I've already plugged an OBD2 reader into it but it doesn't show any codes popping up. "Clearing" the codes this way makes the triangle disappear until I start the car back up though. Anyone know what could be going on here?

 

Thanks for the help, this points me to a lot of relevant info here and on Google. It sounds like the solution will likely be beyond my mechanic ability (I was hoping this would be a burnt fuse I could swap out or something) so I'll have to see if I can track down the shop that replaced the battery and see if they'll find and fix the fault.