German resident permit and living in Netherlands

Question

My fiancé is an Indian National with a German student resident permit with more than a year of validity. Can she stay in the Netherlands, let's say for over 3 months (with her address still registered in Germany) ?

I read that you can travel outside Germany(within Schengen zone) for up-to 90 days. Also, how do they even track these activities? because there is no border control check while travelling by train or even flight from Germany to Netherlands.

Answer 1

Can she stay in the Netherlands, let's say for over 3 months (with her address still registered in Germany)?

As you've noticed, her presence in Schengen countries other than Germany is limited to 90 days in every 180-day period.

how do they even track these activities?

They don't, typically. But if she or her absences were to come to the attention of the authorities for some other reason, there could be some trouble.