I have a student residence permit issued in one of the EU/Schengen countries. As fair as I understand, it allows me to stay without visa in any other Schengen country for up to 90 days in 180 day interval.
I want to ask if and how is this rule enforced and are there practically any possible consequences for overstaying the mentioned 90 days outside of the issuing country.
My situation is that I am currently doing a research internship in Switzerland, which officially lasts 85 days, and I am doing it without any Swiss visa/permit, since my original permit allows me to have such educational/working stays for up to 90 days. After the internship ends, I want to have a holiday and stay in various Schengen countries for up to 15 days in total.
I wonder if any authority can find out that I will be overstaying the 90 days limit. Since there are no border checks in the Schengen area, is that even possible? The only clue that I can think of are hotel bookings: I have a hotel booking in Switzerland for 85 days of my internship, and I will likely stay in a hotel/AirBnb during my trip. As far as I know, they are somehow obliged to check my passport and notify the authorities about my booking. So, strictly speaking, I will have a hotel booking for 100 days, which is more than what the rule allows. In reality, it can be that during my internship I made several short trips to the issuing country, with the total length of 10 days, without interrupting the hotel booking. So, the real length of my "outside" stay is less than 90 days, but that is hard to prove (I can maybe show some transportation tickets).
So I wonder what really happens when I book a hotel and show my passport/permit on registration. Do authorities somehow keep track of my stays? Is it somehow synchronized between different countries?
Did anybody ever have any experience with enforcement of that rule? It seems it is totally unrealistic for it to be enforced, but I guess if some authorities get really suspicious they can find it out, e.g. by requesting bank statements, bookings and checking locations.