Any time an open file is modified, and focus returns to Notepad++, you get the "This file has been modified by another program. Do you want to reload it?" popup, and the current tab is switched to the file that's been modified.
This makes Notepad++ unusable if you want to view frequently changing files and other files at the same time, as you constantly lose your place. Also, even if you open a file from Explorer that opens in Notepad++, the popup may come up, and switch your focus to the modified file's tab, so you don't see the file you just tried to open.
I would expect that the popup should only come up for tabs that are currently being displayed. If files are modified in non-foreground tabs, you shouldn't get the popup until you switch to the tab. This minimizes interruption to the user's workflow.
At the very least, the popup should not change which tab currently has focus.