Beeps while braking in highway

Hello everyone.

Something weird happened while driving on highway. I was driving at 110km/h (about 70mph) and there comes a strong slope, then I put the B mode but the hybrid system started to beep and I did not see any advising light into panel. Then I keep driving but when I push the brake a little more hard than usual, beeps comes again. When I arrived home, I refer to the manual but it didn't say anything about that, does it sounds familiar for some of you guys?.

I have a 2011 Prius.

Thanks in advance.
Regards.