"...some labor economists will say that 5% is the "natural" level of unemployment, reflecting those who are between jobs voluntarily, those who are active looking and those who have unofficially taken themselves out of the job market."

In the US, as I understand it, unemployment statistics include only those eligible to collect unemployment benefits, i.e., those who had a job (an "above-ground" job) until recently, and whose terminiation was not voluntary AND not due to their own malfeasance.

People fired for cause, people whose recent employment history has been in the invisible/underground economy, and people who have not worked at all during the past year or so, are NOT reflected in unemployment statistics.

5% unemployment may indeed be "natural," and it is, after all, a pretty low figure. But it does not include all the categories of unemployed individuals implied above.