What are directories /Volumes/Untitled* (Yosemite)?

I've just started using a new MacBook Pro, which has Mac OS X 10.10.1 "Yosemite" pre-installed. I was surprised to find paths like /Volumes/Untitled in my /Volumes directory.

% ls -ld /Volumes/Untit*
d--x--x--x+ 3 root  admin  102 17 Dec 17:10 /Volumes/Untitled
d--x--x--x+ 3 root  admin  102 18 Dec 12:34 /Volumes/Untitled 1
d--x--x--x+ 3 root  admin  102 19 Dec 11:44 /Volumes/Untitled 2
d--x--x--x+ 3 root  admin  102 20 Dec 11:45 /Volumes/Untitled 3
d--x--x--x+ 3 root  admin  102 22 Dec 07:17 /Volumes/Untitled 4
d--x--x--x+ 3 root  admin  102 25 Dec 22:23 /Volumes/Untitled 5
d--x--x--x+ 3 root  admin  102 26 Dec 10:41 /Volumes/Untitled 6

Each directory appears to be empty, except for an empty .Trashes subdirectory:

% sudo ls -lR /Volumes/Untitled
total 0
d-wx-wx-wt@ 2 root  admin  68 17 Dec 17:08 .Trashes

/Volumes/Untitled/.Trashes:

I am familiar with entries in /Volumes/* being paths to external storage volumes. I have an external solid-state drive (SSD) mounted, and it has its own named /Volumes/mySSD entry. I have a couple of drives mounted from a file server, and they have their own named /Volumes/serverA etc. entries. In previous Macs and previous Mac OS X versions, I didn't see entries like /Volumes/Untitled.

The timestamps on the Untitled* directories are interesting. The first one is from about when I unpacked the new MacBook Pro for the first time. The most recent one is from about when I started up the computer this morning. If I notice changes in the days ahead, I'll update this question.

I do not see other answers on AskDifferent explaining entries like these. There are answers like Problems creating Yosemite USB bootable drive which mention paths like /Volumes/Untitled 2, but those appear to be paths for actual mounted disk devices that happen to have no volume label. I also have a query in to Apple Support. If they tell me something useful, I'll post it as an answer.