Songs in iTunes keep getting "Can Not Be Found" but are buried elsewhere on hard drive if I hunt for them

Despite cleaning up my library multiple times, there are songs there which I have downloaded over the years which suddenly "can not be found" when I go to play or sync them. Apparently they get cast off into the MACbook universe and I have to go hunt them down and reimport them to iTunes. Anyone know why this keep happening? The source of these songs has been direct upload from a CD, download from multiple non-tunes websites, sync'd from my phone... a variety of sources but this has been a recurring problem for years.

I reimport the songs but at some point a dozen other songs will vanish and have to be hunted down and reimported.