why is my freerunner magnetic?

[Robin Paulson]

this probably seems like an odd question, but why does metallic stuff
stick to the back of my freerunner? i noticed yesterday, that it had
picked up a usb connector that was lying next to it. it wasn't near
the speaker, so i know it's not the magnet in that.

the area in question is directly behind the battery connectors (i.e.
away from the battery), pretty much behind the bottom-left corner of
the screen, and is about 30mm x 30mm. it's more noticeable with the
back taken off

i had a look at the schematics, and couldn't see anything that's
likely to be magnetic, so i'm guessing something has become magnetic
at some point since manufacture.

the battery has been out for nearly 24 hours now, so it's something
semi-permanent, not electromagnetic

any ideas?

cheers