My flight to LGA out of YYZ was cancelled and we were told to leave and rebook. Since I had cleared immigration I asked if I had to clear Canada customs on the way out. The agent said no: instead I was given an escort by a agent out a back door who explained it to the officer at the exit. On the other hand, I once was on an AC flight from Ottawa that wound up in an international arrival area and they opened the door to the customs area. We all got in the immigration lines and had to fill out the international arrival cards. "You're arriving from Ottawa?" "Yes" "Just leave".