When I switch back to NP++ (with alt-tab or clicking on taskbar), I'm prompted about a changed file, saying the file was changed, do I want to reload? If I choose yes, the file is downloaded again from ftp and I loose changes. But this is a fake alert, the file doesn't change.
There is something happening in the background that detects fake file change. Note that sometimes I receive the change alert for files that has been opened a long time ago (eg. I'm working with some php files and I receive change alert for a css file, opened when I started the NP++ session, hours ago)