How can I map /mnt/**/* with updatedb to work with (p)locate?

When I use the locate command (specifically plocate 1.1.22), it doesn't seem to include any files under the /mnt directory, i.e. on external drives. The man page for updatedb claims "The whole file system is scanned by default" (which I take to mean /**/*), so I retried after an update, but no luck. I considered creating a symlink to /mnt in a place that updatedb *does* map, but I understand from [this Q&A](https://unix.stackexchange.com/a/650592/610644) that symlinks won't work by default. I also tried setting PRUNE_BIND_MOUNTS = "no" and removing /mnt from PRUNEPATHS in /etc/updatedb.conf, but again this did nothing. So, is there a way to make updatedb or locate work with files under /mnt?