1. Proof?
2. "Secret" agent sounds so James Bond-esque. Real spy stuff. 
1. "On December 10, 2015,
Flynn attended a gala dinner in Moscow in honor of RT (formerly "Russia Today"), a Russian government-owned English-language media outlet, on which he made semi-regular appearances as an analyst after he retired from U.S. government service."
"Flynn sat next to Russian President Vladimir Putin at the dinner, leading journalist Michael Crowley of Politico to report that "at a moment of semi-hostility between the U.S. and Russia, the presence of such an important figure at Putin's table startled" U.S. officials. As part of the festivities,
Flynn gave a talk on world affairs for which he was paid at least $45,000. Flynn defended the RT payment in an interview with Michael Isikoff."
"On February 1, 2017, the ranking Democratic members on six House committees sent a letter to Secretary of Defense James Mattis, requesting a Department of Defense investigation into Flynn's connection to RT. The legislators expressed concern that Flynn had violated the Foreign Emoluments Clause of the U.S. Constitution by accepting money from RT."
Links to a few of the many news stories regarding Flynn, RT, and Putin are contained in the wikipedia RT section on Flynn.
https://en.wikipedia.org/wiki/Michael_Flynn#Attendance_at_RT_gala_dinner2. If you would prefer "undisclosed" foreign agent to the word "secret", I'll happily modify my post to that word. Not all foreign agents are James Bond types. Some are simply propagandists, although you may prefer the word lobbyist instead. The $530,000 was funneled thru a third party which is not unusual when you're trying to conceal connections between two parties.
"By the end of September 2016, Flynn's consulting company was hired by Inovo BV, a company owned by Kamil Ekim Alptekin, the Chair of the Turkish-American Business Council, which is an arm of the Foreign Economic Relations Board of Turkey (DEIK). The company has links to President of Turkey Recep Tayyip Erdogan.
Flynn was paid $530,000 by Alptekin for Flynn's lobbying work. Flynn only registered as a foreign agent with the Justice Department later on March 8, 2017, for the work completed by November 2016. Flynn acknowledged his work may have benefited Turkey's government."
"On November 8 (election day in the United States), an op-ed written by Flynn was published by The Hill, now calling for U.S. backing for Erdoğan's government and criticizing the regime's opponent, Fethullah Gülen, alleging that Gülen headed a "vast global network" that fit the description of "a dangerous sleeper terror network".
At the time, Flynn did not disclose that his consulting firm had received funds from a company with ties to the Turkish government. After Flynn's ties had been disclosed by The Daily Caller, Politico, and others, the editor of The Hill added a note to Flynn's op-ed, stating that Flynn had failed to disclose that he had been engaged at the time in "consulting work that might have aided the government of Turkey", that his firm had received payments from a company with close ties to the Turkish government, or that the company had reviewed the draft of the op-ed before it was submitted to The Hill."
"On March 24, 2017, former Director of the CIA James Woolsey said that in September 2016
Flynn, while working for the Trump presidential campaign, had attended a meeting in a New York hotel with Turkish officials including foreign minister Mevlüt Çavuşoğlu and energy minister Berat Albayrak, son-in-law of Turkey's President Recep Tayyip Erdogan, and had discussed abducting Fethullah Gülen and sending him to Turkey, bypassing the U.S. extradition legal process."
Again, you'll find links to multiple reports in the wikipedia article.
https://en.wikipedia.org/wiki/Michael_Flynn#Foreign_agent
