> Would one expect a big chunk of MuMetal inside a
> capacitive keyboard?

Probably not - last time I looked MuMetal was not cheap
(approx US$65/lb) and for quite specialised applications.
Could be some sort of tin

Magnetic and field-blocking products

http://www.blockemf.com/catalog/emf-electromagnetic-fields_-763_Misc-EMF-Pro
tection.htm

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