A Magic Detector is a device used to detect the magic in an area. It was invented by the mole Sis, the sister of the Professor. According to Sis, he wasn't the only smart one in the family. Sis told that she took version 1 apart to build version 2, but unfortunately the Shadow Realm caused her to lose it, and she tasked Spyro with finding it again.
When Spyro returned it, she build it again and found it crazing out due to Spyro being off the chart, due to him being a purple dragon.