Mongorestore currently restores to "test" db instead of returning an error if mongorestore couldn't find db information from --db option, metadata file, or a directory name. For example:

mongorestore salaries.bson

or

mongorestore ../salaries.bson

This is not publicly documented and has caused some confusion among users.

Link to relevant code: https://github.com/mongodb/mongo-tools/blob/master/mongorestore/filepath.go#L691-L701