Why can my bare PCB be magnetized?
ENIG plating translates to "Electroless nickel immersion gold" (for neophytes).

This means gold plated copper surfaces using nickel as intermediary layer between copper and gold.

Then, if your PCB is slightly magnetized, could be the nickel plating, because nickel is a ferromagnetic material.

https://terpconnect.umd.edu/~wbreslyn/magnets/is-nickel-magnetic.html

EDIT with more info:
With Silver Immersion, the type of silver used is silver sterling: http://www.multicircuits.com/assets/content/files/immersion_silver.pdf

This kind of silver mainly contains silver alloy with copper (7.5%), but there is no industry standard saying that sterling silver has to be this specific alloy, some metal processors will use small quantities of other metals that could be magnetic.

Hope it helps.