Should diskutil list an unmounted drive? diskutil list showed the presence of an external drive.  But nether Finder nor df showed it.  And there was no "oops" pop-up when I unplugged it.

Is diskutil list supposed to show everything plugged in even if not mounted?
From the manual page diskutil for the list verb:


  List disks, including internal and external disks, whole disks and partitions, and various kinds of virtual or offline disks.


So, yes it should list all disks, even if not mounted.

Note however, if you've ejected the disk instead of un-mounting it, it will not appear when using diskutil list. Also, once a disk have been un-mounted, it will not cause a flag to be raise as if pulling out a USB flash drive while still mounted, when disconnecting the un-mounted disk.