Navigation's "previous" trips disappear.

I have an '08 Prius (4). I have had absolutely no problems until now. The past addresses I've programmed in the 'previous' group have all disappeared for the second time. The ones in 'memory' are all still there. The car battery hasn't quit working. This is the second time it's happened. What would cause addresses only in 'previous' to disappear?

While I'm here, I've heard there's a way to disable to back-up beep. I know I'm backing up. I don't need a beep to tell me that! Thanks.