General Question - Will A Detector Find A Mirror?

Best answer is "it depends". According to Wikipedia ...

" Glass mirrors are most often coated with non-toxic silver or aluminium, implemented by a series of coatings:
Tin chloride
Silver
Chemical activator
Copper
Paint"

I know that my detector can detect a rusty finishing nail about the size of a pin - so small that I finally had to sift a handful of dirt to separate it from the dirt. I would think that a small mirror with a metallic backing would return a tremendous signal.

I have a feeling that what you'll find is broken glass and not realize that your "silver coating" has been eaten away, with the detector picking up something else near the mirror. Like the marble I found in my back yard when I dug unsuccessfully after a fairly strong ping that probably was a small nail. Ever see a zincoln that's been in the ground for only a couple of years? Not pretty.

Metal detectors operate by generating a magnetic field which induces "eddy currents". The detector is "nulled" (balanced) to air, which is nonconductive, anything that disturbs this concentric field will generate a imbalance in the magnetic field, causing an auduble/visual signal. Another example are "walk through" metal detectors at airports, same principle, just a bigger coil.

So, it depends on what was used to create the reflective surface/backing in the mirror as to whether it is electrically conductive or not.