Master warning when driving uphill

I was driving my 2009 Prius on a road trip going up a hill in 120 degree heat. The master warning light (red triangle) appeared and a buzzing sound came on, but the car was driving fine and the AC was still blowing cool air. I turned the car around and coasted down the hill. A couple of minutes later, the light turned off. I figured the car was probably over heating, so I drove it a few more minutes down to the coast, parked, and let it sit off for about 15 minutes. After that, I was able to drive the rest of the 170 miles home (including some hills, although the temperature outside was more like 85 degrees) without issue.

About two weeks later (with plenty of in-town driving in-between), I was driving up a hill on a freeway for about 10 minutes in 85 degree weather and the master warning light and buzzer came on again. I got off the freeway and parked about 5 minutes later, and let the car sit off for 10 more minutes. When I turned the car back on, the light was gone.

I took the car into my normal mechanic and the Toyota dealer and they both said there were no codes present, so they couldn't help me. What could be causing this and how can I fix it?

 

The dealer eventually came back and said the oil level was slightly low, so when the car was on an incline, the oil in the pan was sloshing backwards and causing the low oil sensor to go off. I'm not entirely convinced because that seems like something that should be logged on the computer. They were also unable to replicate it on an incline in the shop or on the street. They topped off the oil and hopefully that solves it. I'll update here if it happens again.

Thanks everybody for your help.