All of my MP3s ripped using iTunes have something like this in their ID3v2 comment tag:
000022C0 00001F1C 0000B69E 0000B37D 0001DA2F 00021EF5 000090A2 0000934B 00033934 0002BFAC
What is it for? Why is it there?
No idea, but it’s definitely hex. For the record, I just bought a song from Music Match, and it had a code under “Notes” that said USSM10312427, which is apparently regular old decimal numbers. Maybe if you upload it to [insert your favorite P2P network here] it helps them find out who you are.