French situation above is rather valid for Belgium just as well.
We receive EFT from a Google bank account in Ierland. Google IE does declare their expenses -obviously-. So all any taxman needs to do to hunt down their EU based adsense publishers is talk to the Irish government ...
Easy, simple and efficient: they get them all in one go.
Out here, if you did not declare it as a private tax payer: you're in for a revision (can go back some years (5 I think - more if they prove intend - which they can in this case I suspect). If that income is however not occasional (it is: you get monthly payments ...) or becomes significant: You're in for a show of force of the government. Essentially they'll come hitting you again and again ... first because you're not properly self-employed, then because you didn't declare your income properly being de facto self-employed, then for not paying social security on said income, then for not paying taxes on it ... etc, etc. Essentially all the stuff anybody who's self employed needs to do: you did not do and it all has fines on it for not doing it. Worst of all it come on top of your other income so it falls in your highest tier of taxes (of well beyond 50% for the government).
In the end you probably will pay more than what you earned before the tax man is done with you.
And you can't channel it in a company - since you hid it in your private account.
To me anybody doing this is a fool and playing tax-wise russian roulette.
Moreover any "suspicious" income needs to be reported by your own bank ...
A business getting a monthly deposit of a few thousand EUR: nothing suspicious. A private citizen: quite remarkable as you're not employed by Google Ireland... and your bank most likely knows where you're employed (as that too comes in the same account). Any amount over 5K EUR is essentially "looked" at ... (might be fast - might be automated etc. but it needs to be not suspicious).
You can get lucky and never get caught - or you can be unlucky ...
If you have a business it's easy enough: you simply declare it, it's added to your earnings, you can offset any costs and the profit is taxed, done and it gets taxed at a much better rate than what you'd pay as a private person (most likely self-employed).
Oh, if you do run a business and still collect the money privately - the government has a special rate for you: 309% of the hidden income as a fine. (yep: more than 3 times your hidden income as a fine ... )