The Bronze Key kind of looks like a silver key. Except it's brownish. And it tastes kind of sour on your tongue.
Some monsters happen to be carrying them around. Did they meet a gullible janitor and trick them into giving them his keys? But that wouldn't explain the blood on the monster's axe...
Bronze Keys can be used to open chest sometimes found in random events in a Tier 5 area or higher. These chest can contain one of a few random rewards. Some being a large amount of gold or energy crystals.