No need. The smugglers (or, as I suspect, their bribees in the Mexican Army) have been known to actually go out with metal detectors and radio direction finders to hunt the sensors down.
US Government Pro-Illegals Anti-Minutemen
http://www.theconservativevoice.com/article/14455.html
By Sher Zieve - Three documents that appear on the Mexican Secretary of Foreign Relations website advise that the US Border Patrol must notify the Mexican government as to the whereabouts of members of the US citizen Minuteman Project. The US Border patrol is charged with keeping track of the Minutemen and then giving their locations to members of the Mexican government.
DailyBulletin.com broke the story and writes that a US Border Patrol official confirmed it. The Bulletin quotes Founder of the Minuteman Civil Defense Corps Chris Simcox as saying: "Now we know why it seemed like Mexican officials knew where we were all the time. It's unbelievable that our own government agency is sending intelligence to another country. They are sending intelligence to a nation where corruption runs rampant, and that could be getting into the hands of criminal cartels. They just basically endangered the lives of American people."