If Begonias come back in a different year than when they were first planted, do they grow from a root or a seed?
If both are possible in regard to the specific type of Begonia in the specific location, then it might remain a mystery. What type of Begonia, and in what location?
Those could be wax Begonias, which are somewhat hardy. They usually come back here (Z8b in AL near FL border.) Do not know where the garage in question is, but being next to a structure is a good way to try to "cheat" ones' zone, and attempt to grow plants from a lightly warmer climate. The structure can help keep the adjacent ground warmer, and if roots can establish under it, that would also help keep them warmer. Begonia grandis is even more hardy, but usually has pink flowers. As far as I know, no Begonias are true annuals.