I am guilty of having done this myself. That is, glued large plate glass bathroom mirrors to the walls using silicone caulking. I never had one come loose or fall off, or even tried to take one off, but here is my suggestion:
First of all, wear SAFETY GOGGLES for eye protection! These mirrors are NOT made of safety glass, and can shatter into sharp shards and pieces. Also wear heavy gloves, and a long sleeve shirt (a jacket would be better).
Next, tape the entire mirror with heavy, wide masking or duct tape. If it does break, this will hold the pieces together.
Take a wide prybar, such as a "wonder bar" and work it into one edge (about 10 inches from a corner) of the mirror. You may want to cushion the edge of the pry bar by covering it with a layer or two of duct tape. GRADUALLY apply pressure to pry the mirror away from the wall. This is where the trick will be. You have to apply enough pressure to start breaking the glue bond without breaking the mirror. (unless you don't care about the mirror, then you can apply gradually increasing pressure until the mirror breaks). You may want to cushion the wall from the pry bar with a thin piece of wood, shingle, or even a magazine. If you take it slow, apply only enough pressure to start breaking the glue bond, and BE CAREFUL, you should be ok.
One other point, if the mirror is glued to drywall, the paper of the drywall may pull loose before the glue lets go. You will then have a drywall repair job to do unless you are going to put another mirror up.