Hmmmm... Those of us who with an illegal alien population in or near our communities also have dozens of remesadoras (remittance agencies) at every strip mall.
More than $20 billion was remitted to Mexico last year, $62 billion to all of Latin America and the Caribbean, and most of that's from America. Remittances to Mexico are predicted to be the number one source of revenue this year, exceeding that of Pemex (the state owned oil company).
The Treasury Department was supposed to complete a computer system to monitor remittance shops at the end of this year, but (surprise!) it won't done for another three years.
I don’t question the volume and you could go after remittance agencies (noting that the governmental commitment was made) but I have to believe that it may be harder to trace because there are so many ways to get money out of the country.
Apart from the more conventional methods, an easy way would be to open up an account in the US and process direct withdrawals (by other family members) from that institution in a foreign office.
There are many ways that they can get money out.