I realise I am resurrecting an old thread but here goes anyway.
I found a different temp directory to the one that empties via Preferences, and it could be what you were looking for all those years ago. Adding it here in case it helps someone in the future. I am on Win10.
C: / Users / <Username> / AppData / Local / Temp / Adobe
Seems like the entire Creative Cloud suite stores temp files here, not just Premiere Pro. I found 10GB of After Effects files just chilling here, which I nuked to claim back space on my boot drive.