It's difficult to guess what might be feeding on your cannas because you do not indicate where you are located. Assuming you are in zone 9, we need to know if that is on the west coast or the deep South.
Not sighting the cause during the day indicates the problem likely is a night feeder. This often is slugs or snails depending on your location. It takes a rather large population to do the damage seen. You need to check abound the base of the plants and under any mulch or in crevasses for your problem. Most night feeders like to hide in a dark, moist place in the day.
You might try investigating at night with a flashlight. Good luck!
Kent

Unfortunately Cannas are a gourmet meal for a variety of garden pests. Your best bet is to get up early and try and catch them at it. Major suspects: snails and caterpillars but it could also be weevils. Once you know what they are you can go to your local garden store and ask for a remedy. Unless it is snails Neem Oil is best. With snails just pick em off and smash em!!! Good luck. - Ian

Snails do that to mine, it doesn`t take many with these giant snails we have here. Luckily our chickens LOVE snails
Tally HO!