Normally a warrant is required to access banking details. There are other things the government can lean on as well to access bank accounts on a whim. One of them is notably due to the Patriot Act.

However these days government doesn't really need to directly access bank accounts anymore. There are many other metrics that can be utilized to discern what a person has, where it came from, and where its going money wise. Just a small but extremely common example flaunted wealth on social media, and obtaining retail information wholesale.

Buy a car with cash or bitcoin? It still has to be registered to drive on the road and title transfered. Then now they can monitor the person who sold it for wealth increase. Silly stuff like that easily tracked.