We found the key! Turns out it got caught in a tree. We had shaken the tree the night before in the hopes anything caught there would fall out, but apparently it was well stuck. The daylight this morning made it easier to see them where they were caught.
It's a shame the keys were not spotted before a few members went crawling through the dumpster for them, but at least it makes a good story. And the moral of the story: NEVER THROW KEYS!