In Yosemite, we picked out a campsite around the base of the half-dome fork in the trail. All 3 of us had the rented bear canisters from the ranger station. We were camping near a pretty heavy slope, so after cooking and eating we packed our smellables into our canisters and stuffed them in a bush (to prevent them from easily rolling down the hill). Canisters and eating area were about 30yds away from tents and sleeping area.
During the night, could obviously hear a bear playing with our canisters for a good 30minutes then I either fell asleep or he stopped.
Awake the next morning, without my eyeballs in (contacts) to see a new large, dark rock near our tents. Put one contact lens in and there was a large-dog sized bear staring back at me about 8ft from my tent door. Called out to the guys in the next door tent to get up, bear immediately scattered off.
Checked cans, 2 were virtually unmoved where one was rolled out of the bush just a few feet from its former stash location. Wasn't opened or anything so we ate some breakfast, repacked them and stashed them back in our bush. We were going up to half-dome that day so we had just a couple mini day-packs with snacks, water, jackets so one of the guys left his tent standing and we just stored our full packs inside it.
Coming back from half-dome, I noticed something a little off as we got closer to our gear. Asked my friend if he had taken the tent poles out of the tent as it was drooping down a bit. Then it was all clear as we got closer...
Seems our wake-up-call played with the tent while we were gone. Tore a few holes in it, looks to have gnawed on one of the poles enough to snap it, mesh netting shredded. Everything inside was untouched. Nothing smellable in the tent, we double checked.
Made for a nice story, especially since that was night one. Rest of the time we had to MacGuyver the tent to get it to stand and be rain-proof. Reported it back to the ranger station in the valle. Ranger said the bears in that area are not shy due to the often full backpacking site just below our campsite and most likely was curious. He also said it was likely that that bear has gotten food from that exact tent before in the past and was hoping to score another snack.
Anyway, to answer your question: bear canisters are not smell-proof. They are simply a means to prevent said animal from getting at your food. If they eats your food, they will then associate people=food and become more persistant in their Twinkie hunting.