BPL published a study some years back testing OP sacks using drug-sniffing dogs. The dogs found the OP sacks with drugs every time. Their conclusion was that OP sacks are useless. This contradicts the findings of a test I'd done the year before, using dog food and a dog not trained for scenting.

My own experiment with OP sacks and my dog, using dog food (big motivator for him!) came out as follows: dog food in single freezer bag--even I could smell it. Dog food in doubled freezer bags, I couldn't smell, but the dog went right to it. Dog food in freezer bag inside OP sack, dog ignored. My conclusion was that an OP sack would at least keep the dog from getting into his food when my back was turned. However, bears and most other wild animals have a far more sensitive sense of smell than do dogs!

Also, once you've taken food in and out of the OP sack a time or two, your hands will have transferred scent to the outside of the bag, no matter how careful you are. This will happen with your plastic container, too.

In any case, you need to follow whatever food storage rules are in effect in the jurisdiction in which you're hiking, in this case Grand Canyon NP. Rangers tend to write tickets for those not following the rules.

In the meantime, it sounds as though you have some very happy squirrels!