Exactly what message are the users seeing? As you're doubtless aware the agent is predominantly silent, but it's possible to configure it to prompt the end user for a reboot if an operation requires it (which an AV upgrade might well do.) Maybe that's what they're seeing?
No idea what they are seeing, when we look the message has gone away and as far as I can tell the McAfee software has installed correctly, it reports to the EPO server and gives the correct product information. It may be a reboot message, I was only able to test upgrading from 8.7 to 8.8, we had a few people still on 8.5 when I upgraded so maybe that requires a reboot.
I have it on 178 computers right now and only two people have said anything, so maybe a false alarm.
Found out what it was, it's the Windows "your anti-virus is out of date" balloon popping up as VSE 8.8 was being installed. I do know some people have managed to get McAfee Security Plus somehow, but as I understand it VSE is supposed to deactive/uninstall other anti-virus software when it installs.