As you say, it all depends on which bank it is and what laws bind it - because banks only do what they have to in order to comply with the law, but at the end of the day, all that matters to them is making as much profit as possible, without having to bother their clients too much at the same time.

I'm not sure how it is today, but banks in my country used to have a legal obligation to react if the amount of transactions on the account exceeded a certain amount, and that's how some petty fraudsters were caught because they sold illegal (mostly digital products) and didn't pay attention to such things. Today, these same banks are under the full supervision of competent agencies that see all bank accounts and transactions and can react at any time if they feel it is necessary.