I've done several "transits" of YYZ, 2 with overnight stays, and never had a problem or serious questioning about how long I was staying by Immigration. However, I got a thorough dressing down trying to leave the Customs area with only my rollaboard. The agent at the door had difficulty believing I had only my rollaboard and small backpack and repeatedly asserted very firmly (felt like shouting, but I'm not sure of the volume) that I had to collect ALL my luggage at that point, despite the fact that I told her again and again that what I had on my was all there was.