Which v of RB are you running? Unfortunately I don't think I can offer much help, but can definitely say I was having the exact same issue with v2.0.2 - it would happen very randomly, maybe half a dozen or so tracks every time I opened RB, and never the same ones twice. For some reason RB was adding a few random numbers to the very end of the file path of whichever tracks it felt like - very bizarre. Best I could do at the time was go file by file and relocate them (really just removing the stray numbers at the end of the file path, since none of the files had actually moved), however I haven't had any problems with this since upgrading to v2.0.3.