missing files

Every few days, a handful of my 3,700 songs will go missing, deleted from their respective album folders, for no apparent reason. They are all organized by iTunes in the default location. I take serious care of my library and have no idea why RANDOM files go missing, and it seems like it happens when I sync my Nano. I re-did my entire library last month (deleted everything, sent all to the recycle bin and then emptied it, re-added from an external hard drive), losing ALL of my play counts, just to make sure every song was accounted for in the library (since so many just "disappeared" I needed a complete wipe). Even now with a fresh library, songs are slowly disappearing (I got the sync error today that "18 files were not added to the iPod because they could not be found") There is no reason why any of them should not be there, especially since 3 of them I added just yesterday and played them on the speakers while I was cleaning house. The folder structure for each of these missing files is still there (ITUNES MEDIA> MUSIC> ARTIST> ALBUM>) but no song in the folder. What is going on?! Thankfully I back up my library in several locations on multiple external hard drives. I had to start after iTunes kept stealing my music!! Any suggestions as to why things are disappearing?