I noticed it a few days ago, but realized it for sure tonight. Last night I was organizing my library, and had 60+ built to spill mp3s. I changed the info of some of them to make them more uniform, and then tonight I open up itunes and I have only 4 built to spill mp3s.
I just now added 5gb of music, they successfully copied over to the itunes directory, and the total amount of music in itunes was about 15gb. I restarted the app and now it's back down to 10.
They're not in the trash, or anywhere that I can find. In fact, now that I look, the size of my library has gone down from 11GB to 10GB. Have you ever heard of this before?