Hi,
Been looking into an issue with my Sister's Win10 PC - last night Windows Action Center started saying 'Windows Defender scanned your PC at (Time) and took action against malicious software'. However when she checked the quarantine logs there was nothing there and when she ran WD again got the usual 'No threats were detected' message.
The message has now been alternating but I have done a search and found that this message has been appearing elsewhere over the last few days. Sure enough, I've just run again this evening and although the malicious software message has appeared again WD itself says it found no threats.
Looks like Microsoft trying to be too clever for their own good...