Hi Matt,
You are correct. Each printer queue that is created has a package within the ~/Library/Printers directory and within this package is the PrinterProxy file. Opening this app presents the user with the GUI of the printer queue, which of course stays open until closed.
Since the file was deep within the package, I couldn't see how the OP would be given a reference to the file. However, when I checked the CPU resources, via iSlayer's iStat menubar utility, it showed the PrinterProxy running. Opening the Activity Monitor did not show the PrinterProxy directly, but inspecting the relevant printer queue did show the proxy as a child process.
So, I would have to assume that the OP is referring to the fact that the printer queue stays in the dock after printing, this showing the proxy as a running process. If this was the case, then the good old control-click on the Dock icon would provide the means to select the auto quit.
If there is no printer queue showing in the dock, then I have read where older apps could still be calling the proxy within their own package located some place other than the home library. Reading an article on macosxforums referred to an Epson printer doing just that. The fix was to reset the printing system and/or trash the contents of the vendors directory in ./Library/Printers. This is what I assumed could be occurring for the OP, having not seen any other message related to the PrinterProxy previously. But as I have done many times before, I may have jumped too deep into the issue when the answer could have been a lot simpler. Thanks for the info.
Paul
