Park brake issues 2004 Prius

I left my car in Park for a few minutes in a carpark today and a warning light came on that Id never seen before. "The park function is abnormal" or such.. and "to drive to a level surface and use the foot parking brake." Then an icon appeared in the top left corner of my LCD display of a car silouette with a ! through it and I also had a red warning triangle on my dashboard with similar !. Turning the car on and off did nothing. I drove away and all worked fine. I parked again and on return to my car to drive off all was well again!! All warning lights were now gone. All I can imagine is that the 40 degree slope I was parked across somehow upset the car's sensors? Do I need to see my Toyota dealer about this? Thanks in advance.

 

Hi and TY seilerts! I tested my 12 v battery and found it reads 9 v under load..now I dont drive the car often and for long distances so could it be the battery needs to be charged on a long drive? On the other hand, 6 yrs is pretty old for a car battery isnt it?

Cheers,
Lu