Decoherence Residual

We demonstrate the existence of a universal, structurally unavoidable non-Markovian correction to effective exponential decay laws in open physical systems. The result is derived without assuming any fundamental dynamics, background time, spacetime structure, or microscopic model. The correction emerges solely from three generic ingredients: (i) the non-closure of correlational descriptions, (ii) projection to observable subsystems with unavoidable information loss, and (iii) spectral truncation of the induced structural operator. Under these minimal and representation-independent conditions, any effective exponential decay is necessarily accompanied by a long-time algebraic correction of the form $P(t) \sim e^{-\gamma t}\left(1+\alpha/t+\mathcal{O}(t^{-2})\right)$, with a fixed exponent and sign that cannot be removed without violating structural consistency. This result is independent of gravity, cosmology, and specific microscopic dynamics, and applies generically to quantum-coherent, weakly dissipative regimes commonly described by Markovian semigroup models. The prediction provides a direct, experimentally testable signature distinguishing strictly Markovian descriptions from structurally non-closed correlational frameworks.