Must list auto body repairs on return to Canada??

We cross the border regularly.We always declare our purchases. I never used to declare gas until one day I pulled in and did not realize I forgot to close the little gas door. The customs fellow asked what we purchased and I told him. He then said did you purchase gas? I said yes and he said good answer and then pointed to my gas door. He asked why didn't you tell me you got gas.I said I did not think I had to declare it. I always make sure I do now.