How come? AFAIK there is no big problem in Denmark (which has the flexicurity approach) to collect social security payments. All employees have a person number registered at the civil registration system. Companies pay the social contributions and tax withdrawals with the PAYE system.

The only exception is perhaps "shadow economy" of immigrant-dominated areas in some cities where rule of law is weaker and restaurants and other local businesses may use undocumented employees. But these cannot be very big business.