It is a marvel of modern technology that the U.S. government can track a cow born in Canada almost three years ago, right to the stall where she sleeps in the state of Washington, and determine exactly what that cow ate. They can also track her calves right to their stalls, and tell you what kind of feed they ate.
The same government is unable to locate aliens wandering around in their country, including people that are trying to harm us.
Now, some of the tracking technology involves implanted RFID tags, bar codes, tattoos, or brands -- things that civilized countries do not do to people.
Solution: Give every visiting alien a cow as soon as they enter the country.