there are no unit files, but there are units:
[plo@plo-air ~]$ systemctl --all | grep plym
plymouth-quit-wait.service masked inactive dead plymouth-quit-wait.service
plymouth-start.service masked inactive dead plymouth-start.service
On my system, those units do have unit files, and they're provided by the Plymouth package. I couldn't find any reference to "plymouth-quit-wait" in the systemd source code, so those strings have to be coming from somewhere. Are you sure you don't have a unit file at /usr/lib/systemd/systemd/plymouth-quit-wait.service ?
I don't see any plymouth-related services or unit files hardcoded. I do, however, see a NEWS-related entry from the v186 release: http://cgit.freedesktop.org/systemd/systemd/tree/NEWS#n1775