Already scanned, thanks, GG. Nothing found.
From what I've been told I'm pretty sure that the System 32 directory was used because of human mistake under pressure, rushing to finish something before the Nth time of being called to the dinner table - when it would have been better to wait. Most of us have been there, so I sympathise! My money is on clicking on the wrong part of the breadcrumb at the top of the Save As dialogue and pressing "Save".
Anyway; back to the thread; I still can't work out what the trigger is. I now know though that my company laptop (W7 Enterprise 64bit) and both my and my friend's home PC (both W7 Professional 64bit) all behave the same weird way.