I always go to the US and return to Canada by private vehicles. Even if I go abroad, I go to US first and fly to another country, and come back to the US, then drive back to Canada. I used to be a Ph.D. student in the US and that's why I always try to find time to go back to school for meetings with my advisors.
Sometimes the border officer just checked my PR card and let me go. I do no think the officer scanned my card or passport in some trips. Although I did report these trips, I just wonder if I will be given hard time in the interview.