STR:
Preferences -> MISC.

Open files A and B.
Move B to the other view.
Activate A.
Externally modify and save B.
Activate B.
Result:
NPP does not alert the user on B's changes.
NOTE:
Setting "File Status Auto Detection" to "Enable for all opened files", NPP does prompt as expected.
Notepad++ v.8.5.7.