I didn't think about the sponge thing. Seems like it would affect all of the ink. But only the light orange inked pattern is mostly affected. The black ink, the red ink, and the yellow small "10"'s around the torch do not seem to be affected at all when I look at it in person. The color shifting torch is partially in the white area is also not affected.
This makes me think of a couple scenarios:
--It happened at the BEP, between the stages of production. Where something foreign was spilled or introduced to the original sheet. And it went unnoticed through the other over-print ink passes and only affected the orange ink.
--The note was introduced to a specific solvent that only affects the orange ink , outside of the BEP. But I'm betting 99.9999% of the public wouldn't have intimate knowledge of the specifics of the ink composition because it'd be a security risk, for it to be public.
I dug around the internet some tonight and can't find any science experiments that schools have kids do that would cause this like turning a copper cent "golden" by heating a zinc coated cent, and etc.