Changed by Another Program: Prompt.
Monitor Interval: 5.

I tested further and found this problem happened only when the content of the file(s) was changed by McAfee VirusScan itself. If I use Notepad2 to change the content of the file(s), EmEditor will prompt. But if the content of the same file(s) is changed by McAfee VirusScan, EmEditor will never know, no matter how long I wait.