I'd have a good dig through the backpack: to see if there is anything that identifies a store. There may be a receipt, something still in package that has a price sticker, or something hard-to-find that can only be purchased in a couple of places.
If you find something that identifies a smaller store, I'd phone them and let them know what you've found. The owner of the backpack might have started to replace things and probably would have said something to a salesclerk or cashier about having his/her backpack stolen.
You might also like to put an ad up on Edmonton Kijiji. Don't post a picture, of course, just the details of the find.
Nice to see that you're so honest, GMac
If figure that if you make a good effort, and haven't found the owner by the time the thaw comes (if it ever freezes, which almost seems questionable at this point), you can call it yours.