Yes, a motorplan car can be repaired "under the radar" at a private, non BMW Approved repairer to avoid getting comments on the system. But...a BMW dealer could find out if it goes in for a particular repair/service and the technicians spot a problem or sign of accident repair. What happens then is the car is suspended from Motorplan immediately pending a full check. Highly unlikely that an accident car passes the check without alarm bells...they are thorough! Sometimes, the car is reinstated on motorplan with comments were its highlighted that certain aspects of the car (that was repaired) is not covered by Motorplan anymore.
However, this is only for major repairs....not for slight fender benders that requires a purely cosmetic repair. Those don't need to be reported on motorplan if the repair cost is under a certain amount.
Just speaking from experience.