>First they need to show proof that you are staying there for longer than allowed. How do they do that?
At least here in the democratic republic of Hungary: Cell network subscriber logs, ATM/POS flags, the passport number you enter when getting your public transit pass, the filed invoice the phone repair shop fills out when you get your battery replaced.
If you're a backpacking 20 year old it's very unlikely anything will happen to you, but if you're running a business or a "digital nomad" making FAANG salaries you'll get a rude awakening one morning with a knock on the door.
If you're making FAANG salaries you just register and be done with it.
The whole discussion is not very relevant and is a nitpick. The point is that anyone in the EEA/Switzerland can freely move to Portugal (or any other country) and that unless you get onto the authorities' radar very negatively you are safe. The vast majority of people will have an income and/or look for work and will be entitled to stay, anyway.
At least here in the democratic republic of Hungary: Cell network subscriber logs, ATM/POS flags, the passport number you enter when getting your public transit pass, the filed invoice the phone repair shop fills out when you get your battery replaced.
If you're a backpacking 20 year old it's very unlikely anything will happen to you, but if you're running a business or a "digital nomad" making FAANG salaries you'll get a rude awakening one morning with a knock on the door.