Can Magnus-based sensors be used to detect the proximity of metal to the sensor

Yes. For example, a sensor tag can be positioned so that the nearby metal alters the impedance of the tag antenna. This will be detectable as a progressive change in the Sensor Code as the metal gets closer to the tag. However, there are many dependencies based on the specific user application. For example, is the sensor being used to detect the proximity of two pieces of metal relative to each other, or one metal object relative to one insulating object? Other dependencies may include the sizes/thicknesses that are acceptable for the sensor and where the reader antenna may be placed.