Network disconnect causes leftover directory under /Volumes
Not an answer, just to let you know that I cannot emulate your problem in a similar setup, and that you might have a problem.

I have an iMac with 10.6.8 and a MacBook with 10.8.1. Sometimes I forget to eject the connected volume on the iMac before I shut down the MacBook, and then the mounted volume remains as an image on the Desktop and in the sidebar of any open folder. I then simply click the eject-symbol in the folder sidebar or right-click on the desktop symbol and tell finder to eject the volume. It disappears. Now, looking in my /Volumes (in Finder as well as in Terminal), there is no remaining network volume. Maybe ejecting it did the trick.

Now, if I do NOT eject the remaining volume after I shut down the MacBook (or put it to sleep) and then boot (or wake) it and connect it again, the remaining volume icon on the desktop disappears and a new one appears. If I check under /Volumes (in Finder and Terminal), there is only one connected volume. (I did not shut down the iMac or do anything, except shut down the MacBook and start it (or put it to sleep and wake it) and connect it again.)

I do not get a Finder notification, that the drive is no longer available, either. If I try and access the disconnected drive by double clicking on the icon on the desktop, I can even browse its contents, until it automatically disappears when I reach I point where I have not been during this connection (open a subfolder that has not yet been opened).