Its definitely the system detecting no driver in the driver's seat. I have done this on numerous occasions. I would have the "Auto Hold" feature on while in traffic, reach for an item in the back seat or reach for an item on the passenger floor, traffic would start to move, I would position myself back in my seat, press on the throttle and get the "chime" "parking brake activated". So I would have to deactivate the parking brake then resume driving. I have tested this out several times and this is definitely what my issue was. Stay seated and this should solve the issue. Great safety feature though. Hope this helps...