The IRS will only be interested in someone if they aren’t laundering their money. Spending a bunch of money with no provenance is what is going to make them look at them.
The whole point is to pay taxes on it and make it look legitimate.
Other enforcement agencies may be interested, but they can’t really do much without enough probable cause to get a warrant.
There are no public books for them to look at, they don’t know their sales, they can’t tell how much they are spending on products or labor, or even rent, unless they already have enough information on them to get a judge to let them take a closer look.
And in this day, it doesn’t even really need to be a cash business. They can use their ill gotten gains to buy prepaid credit cards, then use them.
I’m not sure how much more I should go into, as discussing how to commit a crime is against board rules, and I also don’t want to make it look like it is something that I am doing, but really, just a bit of common sense would take a money launderer a long way.