The Magical Field
The Great Gates used to be strong. They used to be mighty. They used to be impenetrable. Alas no more. After thousands of years of spell casting, they have become ever so slightly leaky. Now ether constantly diffuses through the gaps and spreads out into the real world. Such ether is drawn up in any spell cast as it is the most easily accessible so monitoring it will tell you of any spell caster in the area.