From the updatedb manpage:

updatedb is usually run daily by cron(8) to update the default database.

Also:

To create a private mlocate database as an user other than root, run

    updatedb -l 0 -o db_file -U source_directory

Note that all users that can read db_file can get the complete list of files in the subtree of source_directory.

Because updatedb needs to look at your system's private parts. Try

    find /. | grep [string]

which will look everywhere for [string]