Does the U.S. monitor who is on foreign flights?

It is unlikely that US officials would be advised either by Mexican officials of your departure or Argentinian officials of your arrival. As a US citizen, you don't need a visa to enter Argentina, although changes earlier this year to the Argentinian immigration laws may subject all arriving passengers to new, and added, scrutiny. Its Customs and Immigration officials now access both the Advanced Passenger Information (API) and the INTERPOL Criminal Information System databases as part of the country's improved border security measures.