Temporary Brake Warning light

I just bought a second hand '03 Prius in great condition from my sister. She never had any problems with it. The other morning on my drive into work I slowed for a right turn onto the highway and then gradually accelerated on a straightaway when my brake warning light came on ("BRAKE") for 3 seconds and then blinked off again. Nothing else felt wrong or out of place and the light didn't appear again for a week or so. Then it happened again--same time in the morning, same right turn, same spot on the straightaway. This time I took my foot off the gas, but the light stayed on for 4 seconds and blinked off. A few more days went by and it happened again--all the elements were the same, this time it stayed on for 5 seconds and then blinked off. Has anyone ever experienced this issue? Is it just a coincidence that it happens under the same exact conditions each time? I'm concerned that it's the brake light too. Any advice?