[iudqnolq]

One tool is parallel construction. First they find out what you did through an illegal/classified method, then they use the benefit of knowing the answer to construct a way to figure out the same information legally.

For example, an agency illegally taps your phone and find out you'll be driving with something illegal in your car along a certain route. The agency then tells some state troopers to notice $your_car driving unsafely at $location and pull you over for a routine traffic stop and search your car.

It's unlikely they'd take even that risk on a tax case, though.