Buddy system
A buddy system pairs workers so that each person has a nominated colleague who knows where they are, checks on them, and raises the alarm if something seems wrong.
The buddy system is the oldest safety control there is, and at its best it brings things technology cannot: judgement, context, and someone who notices that you sounded wrong on the phone, not just that you missed a check-in.
Its failure modes are equally human. Buddies get busy, go into meetings, finish shifts, and assume silence means fine. The riskiest hours - evenings, weekends - are exactly when the buddy is least available. And the system doubles the coordination load: every solo worker needs a reliable counterpart, every shift, without gaps.
The strongest designs keep the buddy and add a system underneath: automatic check-ins that escalate to a monitoring centre when both the worker and the buddy miss them. The buddy provides care; the system provides certainty.