Sewer vs signal ground question

[Rob Strand]

Can the currents in the enclosure in turn induce Eddy currents in the enclosed circuit?

The eddy currents arrange themselves so as to counteract the external field so they always act to reduce the field at the enclosure surface.   That doesn't mean it blocks the 100% field.    How much field gets through depends on the skin depth, the frequency and the thickness of the enclosure.   50Hz needs 10's of mm of copper.

Eddy shielding is completely different magnetic shield using iron or mu-metal.