hmmm... from all the Googling I did, the most common is that you wear away all the material. If there is enough knockback, the backing plate can slide between the caliper and the brake disc falling out. I am assuming you would hear this, as it should be loud.
Another way would be if the pad delaminated with the same failure scenario.
Also, I guess if the clips on the end are not holding the pad in place and there is enough knock back, the worn pad could fall out, as above.
Does the pad that remains fit snugly in the outside of the caliper or loosely?
Did you notice anything from around the time you lost the pad? Do you know when you lost the pad?
Has any work been done recently where it would have been possible that someone just forgot to put them in?
You got anything else, @
Ultramaroon?
Most of the searching I saw was from worn or failed parts but it is a common enough failure scenario.
In the short term, I would just go to NAPA/O'Rielys/AutoZone/Advance and get some stock OEM pads. Put two new ones in there and see what happens (keep note to keep looking). Take some left/right corners and then hit the brakes. See if there there is a feeling of knockback. Inspect the caliper and piston movement before to place in.
Try to determine if it was isolated (wear), a caliper failure, or a bearing/hub failure. Go from there.