In Spain, as of right now, you have to declare your bank accounts (even with $0 balance), any crypto you own (exchanges, wallets, etc), and probably broker accounts too, but not 100% sure about the last one.
Besides all that, there's the 720 Model as
@santosg said, which you have to file if you have over 50k Eur in assets such as money in bank accounts, funds, stocks, property, etc. (Physical gold, jewellery, cash stored in a vault, cars, art, wine and foreign-registered boats are excluded)
Fines for not complying are absurd, starting at around 5k Eur for not declaring your accounts (with a minimum︀ of 10k), 1.5k for declaring them late, and around 10k for each category on the︁ 720 Model. You can also face up to 150% of the value of the assets︂ in fines if you never paid taxes for the assets either. The legality of the︃ 720 Model is questionable and you'll probably be able to take them to court and︄ maybe get a smaller fine, but it'll be a mess and waste of time.
Some of the info above may not be 100% accurate but all I'm saying is if︅ you're a tax resident in Spain and you're doing shady things with a lot of︆ money, get out. If it's low amounts I doubt they'll care.
I don't know about︇ Portugal and Greece, but they will probably both give you big fines if they find︈ out you haven't paid taxes on assets you have in other countries.