I have this problem too. I don't know if it's related but in addition, a couple of scheduled Quick Scans ago, the windows also said Threat Detected! Clicking the 'details' button had no effect. However the scan history showed that no threat had been found and there was nothing in the Virus Chest. I mildly panicked and ran Malwarebytes, but that didn't find anything. I then ran another couple of Quick Scans and both came up clean, as did the following scheduled one. Just an aberration?