Readiness drift is the slow, often invisible degradation of performance against the unit's own calibrated baseline. It is not the same as variance between units or comparison against fictional benchmarks. It is variance against the unit's measured history.
Most readiness systems do not fail loudly. They fail quietly. And then incident finds them. Drift detection turns the silent failure mode into a surfaced signal — with a finding, an owner, a due date, and a closure record.