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Update - It seems to be the case that these files are being created by Avast. After a tense Avast uninstallation and reboot, I returned to the AppData/Local/Temp folder to discover no new appXXXX.tmp-shm files in there. A second reboot confirmed this further. A new set of appXXXX.tmp-shm files re-appeared in there as soon as Avast’s UI loaded after reinstallation.

Whilst this makes me a lot more comfortable knowing that the greatest likelihood is that these files are created by Avast - plus a fresh installation helps to allay my fears that Avast had been somehow compromised, I was wondering if anyone knew anything more about these files?