A few tips, nothing solid I'm afraid.
Are you logging onto the PDC as the same user as the exchange box?
I use MS proxy server, and one of the things I notice (I haven't analysed it deeply) is that you get 'your own' file cache regardless of where you log on. So if you are logged on to the PDC as a different user, you may see a different set of temp files.
If this is the case, maybe if you log on to the Exchange box with the same user name as the PDC, you will see the correct files.
Having said this, I haven't actually worked out the mechanics of how it identifies the different caches. I do know that this kind of stuff doesn't always appear as 'files' in the temp internet file directory, it is normally contained in a file called index.dat.
You could scout around your network for any files of that name, and that might solve the mystery. You might find the file is hidden - to see it use attrib, rather than dir. Provided you don't mind losing your cache, you can safely delete the index.dat file - it won't do any harm. If it's hidden or has system attribute, just type
attrib -h -s index.dat
and you can do what you like with it. (sorry if I sound like teacher! I'm sure you know all this).
Finally, just a quick idea, you could use Internet explorer to delete the temporary files - that might get rid of it anyway.