After successfully swapping the CD-ROM drive in my iBook for a CD-RW drive (using the very helpful guide purchased from this site), I found a small rectangular magnet, about 1/2" by 1/4", that had attached itself to the CD-RW case. It has adhesive on one side, but I could not find any attachment point where it might have come from. Everything seems to be functioning properly. Any idea where this might have come from, or what it might be for? This is a used iBook, and it was apparent that the previous owner had opened the case before. Could it just be some surgical instruments left behind?
Thanks.

If you remove the keyboard you will see a screw located in the top center that attaches the top case to the the notebook. That magnet covers the screw hole.

FWIW, the magnet you are describing is one that goes under the topcase, just below where the "->" key is. The magnet that goes under the keyboard is a round magnet. If you look under the topcase in the area I described you'll see a little ridge to help position the magnet.