It can happen if there is a momentary interruption in protection for whatever cause. If the McAfee SecurityCenter is green and says it is protecting then everything is fine, especially as you say the MVT found no issues.
If it starts occurring more frequently then maybe run some extra tools to be sure there's no malware, see the last link below, or contact Technical Support by phone or online chat (free) if you suspect the software is corrupted, link also below.
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The best of the season.
Thanks, you too. ;-)