A foreigner is assumed to have moved his principal place of residence outside the Netherlands if he has been outside the Netherlands for six consecutive months. After this period the Immigration Service can withdraw the residence permit.
A highly skilled migrant can stay for longer than six consecutive months outside the Netherlands for the purpose of work, and keep his highly skilled migrant residence permit in the Netherlands. Required is that the highly skilled migrant stays registered in the Dutch civil registry and continues to qualify for the highly skilled migrant salary threshold.
The IND has recently published this policy rules (Stcr. 2014, nr. 8489).