Is it possible for a mcl tear to go unnoticed until an arthroscopic surgery?

Not typically: The mcl ligament is not technically inside the knee. It consists of two parts, and neither part is really visible with arthroscopy. However, it is sometimes not picked up if the surgeon did not perform a thorough history and examination. Further, an mcl tear is often made worse during the pulling and torque involved in knee arthroscopy. Please inquire with your surgeon.

Not typically: Your mcl is one of the few ligaments that if you injure it doesn't need to be repaired. It would be unusual to find an mcl tear at arthroscopy, particularly since you can't really visualize the ligament with the camera from inside the knee. Sometimes during arthroscopy the mcl will undergo 'relaxation an release' as part of the surgery to better evaluate the medial knee.