From what I gather, according to the Dutch National Institute for Public Health and the Environment, they started sampling on a weekly basis from the 17th of February [1].

To paraphrase, they didn't find anything the first two weeks, but samples from the 2nd, 9th and 16th of March showed signs of the virus, four days after the first person in the Netherlands tested positive for the virus.

They mention that they have applied this technique for the norovirus, measles and others before.

1. https://www.rivm.nl/nieuws/nieuwe-coronavirus-aangetroffen-i...

Thanks, interesting! Shame they didn't start in the south in februari, could have made a difference.