Windows Defender on Windows 8.1 pro
From a background maintenance run, Windows Defender detected:
PortableApps was not running at the time. Initial scan of .paf (with WD) did not alert. Scan of .paf (with WD) after the warning also showed no threat.
99% chance that it's a false positive. Nevertheless, I've had to take the precaution of removing it until further notice.