The discovery Thursday came after the manager of the self-storage business in Clearwater called a woman because she was behind on her rent. The woman told him her grandmother was in the storage unit.
Police found the remains inside a blue coffin.
The Tampa Bay Times reports the woman told police her own mother told her about the remains as she was on her deathbed last year.
Police recovered a death certificate for the woman, who died in 1995. Medical examiners told police the grandmother's body had been properly processed for burial but it was unclear why she was in the storage unit.
Police say they don't believe she died under suspicious circumstances.