Red triangle + VSC/ABS while driving uphill

Hi folks, hoping to get some insight on the following issue w/ my Gen 2 / 2007 base:

While driving up a steep hill for about a quarter mile, the red triangle + VSC/brake warning lights came on. No loss of power or other noticeable issues, and car drove normally until I could pull over a mile later.

Turned car off and back on again. Red triangle remained, VSC & brake lights were gone. Basic scan showed no new codes (besides ever-present P0420). I cleared codes, and red triangle disappeared.

A few miles later on the next steep hill, red triangle, VSC, ABS lights came on again (along with usual check engine). Performed same series of steps two more times with the same results.

Some addt'l info:

• Odometer: 124,572

• I bought the car 3 months ago. Previous owner was removing/reinstalling catalytic converter due to theft concerns. He put it back in a few hours before I picked it up. I drove 5-10 miles before check engine came on (code P0420). It's been on ever since.

• New hybrid battery was installed 6 months/1,500 miles ago. It's under warranty for two years/24K miles.

• AC running when warning lights came on (caused no change to AC).

• Inverter coolant level is normal and shows movement when car is on.

• Oil, brake fluid, and radiator coolant levels all normal.

• Battery recharging normally after idling.

My next step is to get it scanned for any Toyota-specific codes at my mechanic Monday morning. In the meantime, would really appreciate any thoughts from the community. I tried to include as much info as possible, please let me know if I left anything out

Thanks!