I see you are zone 5-6 but not exact location, I am in zone 7 Baltimore County and have only one survive over the winter. Spartacus. Sorry I can't be more help. Steve.

I am in Zone 6a. Indeed, it is very unusual for your dahlia to have made it through the winter!

I had a couple tubers out in a cold frame this spring, one night the temps dipped just below freezing and that was enough to kill them.

However, there is much to be said for "micro climates". Your zone (5 or 6) is reflective of what is typically found around your area on a large scale. It is possible to find warmer "pockets" that do not experiance the same temperatures. Classic examples are along foundation walls of a heated structure, if enough heat radiates from the building, it could warm your soil enough to be the equivalent of a much higher zone.

Another possibility is if you heaped a lot of material on top of the ground, and it began to compost, the pile may have heated up / sheltered your tuber enough for it to survive.

Enjoy the flower, but if you want to keep it for next year for sure, I would dig it out in the fall!!