Weird thing happened. I realized that the song I just listened to on my iPod right before synch is nowhere to be found after the synch. Not even in the Recycle Bin! So I went further, restored library with a 2 days old backup, searched for missing files using If(IsMissing(), The file is missing., The file is here.) expression. Sure enough about 20 files are gone. And these are rated 2,3 and 4, there is no way I would delete it myself. So it lookes like MC erased the files and updated the library.