I’m no expert, I can only offer my opinion, for whatever it’s worth…
You can find work in Spain with no work visa, yes. However, it is illegal and so you obviously won’t be paying taxes on that in Spain. Nor will you be paying Social Security, so you won’t get any benefits.
The U.S. wants you to file taxes wherever you are in the world, whether you have official residency abroad or not. They don’t care. They just want your money.
If you earn money from the U.S. and/or anything legal, I would DEFINITELY report that to the U.S. regardless of whether you have already paid taxes on it in another country. The U.S. has tax treaties with some countries, including Spain, to reduce the double tax burden.
Again, I’m no expert, and I don’t want to encourage anything illegal, but…
If your money is coming from illegal, under the table sources in Spain where no one has any official record of you actually working there, why would you report that to the U.S.?