You left canna rhizomes in the ground overwinter in zone 5 and they lived? Were they very close to a south facing foundation of your house? Kinda rare to have unprotected canna in zone 5.

Are you sure it is canna lily? Seeds don't really sound like what I have. Seeds are large, but I don't usually have many. Can you possibly post a picture?

there is no purple canna. I am wondering if by any chance you might be talking about glads. Also, it is possible (though not very likely) that your plant is in a warm enough area of your garden and that is why is has come back. Also, if you have protected with heavy mulch, that might help the situation.

As pic will be really useful in giving you definitive ansnwers.

I'm thinking you may have a calla lily instead of canna. Does it look like the ones on this website?

Here is a link that might be useful: Calla lily