Just a came across this Terminology like to 2 days ago and various discussion led me to do some research around it.
A search on Google came back with different items:
Second and third item are videos on Google. First one „Illuminati: The Hidden Agenda for World Government An interview with Norman Dodd“ and another one called „The Hidden Agenda: The Fluoride Deception“ about „Monteith goes over the history of fluoride, its use, its dangers and its promotion over time. Why something that is rejected by so many nations is promoted here in the USA. Learn about the Hidden Agenda behind the use of Fluoride, who’s behind it and the real purpose behind its use. „
The one that i am after is the one on wikipedia „Hidden Agenda“ . Which defines the term in more detail:
an idiomatic phrase used to refer to the subtext which discreetly drives a conversation, meeting or other activity
in a phrase context it also talks about
a term used exclusively by the Liberal Party of Canada led by Stephane Dion. The phrase is used to target Prime Minister Stephen Harper and the Conservative Party of Canada about any possible negative tactic that they may bring upon the Canadian public, without warning, that is sure to bring uproar and widespread criticism across the country (i.e: criminalization of abortion, reinstatement of the death penalty, overturning the same sex bill, etc).
Answers.com defines it a little bit more precise:
An undisclosed plan, especially one with an ulterior motive (for non native Germans: der Hintergedanke) motive. Unannounced objectives, needs, expectations, or strategies of a person or group when participating in an activity. Since individuals keep their agendas secret, one has to rely on minimal clues to determine what others are thinking.
So if you come across somebody has a „Hidden Agenda“ try to find out as it might put your on agenda at risk.