Belgium - how easily can police ask for bank account traffic

[Psyduck]

How easily can the police dispose of information about your bank account traffic? Do the banks respect our privacy and refuse to give any information if there is no strong motivation? Is the procedure similar to acquiring a house-warrant? What if they ask me to show my bank account traffic because they suspect something is not right (for example, when once in a while there is a duck in my mailbox -- which is suspicious, but legal according to the law...). Can I refuse to show my bank account traffic? What rights do I have when there are only suspicious things in my mailbox (that are legal according to the law)?

(Location: Belgium, Europe)