If a hazard hasn't been detected then it can't be assessed or controlled?

If a hazard hasn't been detected then it can't be assessed or controlled?
Not exactly. You might not know about a specific hazard but you can be aware of a general class of hazards and assess and control that class of hazards.

For example, I may not know of a specific rock in the road to Half-Moon Bay. However, I do know there are cliffs next to the road and that rocks can fall onto the road there. So I asses the risk (did it rain recently? High winds today?) and take appropriate action (go around curves slowly). So even the rock I cannot detect (due to curves in the road) can be assessed and controlled.