Well, Christmas day wasn't bad until....

Returned home from in-laws house to find the approx 5 ft by 3.5 ft bathroom
mirror had come loose from the wall and smashed to pieces on the floor. It
had been held in place by eight dark patches of a (now hardened) adhesive
tar-like substance. Held it for 11 years, but I would think this would never
happen unless it wasn't affixed to the wall properly. There was no residue
that I could find on the mirror from the adhesive, so it looks like the
mirror broke away from the adhesive (versus the adhesive shearing or pulling
away from the drywall).

I'm going to call a glass man out to replace... but is there a better way to
anchor the mirror to prevent it from happening again? And is there a way to
check to see if other mirrors are about to fall also?