Flynn had no reason to suspect he was under surveillance as he never was doing anything unethical or required secrecy......
What we are all observing is that elements of our own federal government are every bit as EVIL as elements of any other government we designate as a COMMUNIST ENEMY threat.
That's the whole point. In that particular case he WASN'T under surveillance. It was the Russian ambassador (Kislyak) who was under surveillance, and U.S. intelligence recorded Flynn on the phone with him.
Flynn knew this better than anyone else ... which is why he asked the FBI dopes (when they interviewed him) why they were wasting their time with him, when they could just check the records from the Kislyak surveillance if they wanted to know what he discussed with him.
What made this reckless on Flynn's part was that he was carrying on this conversation on a cell phone call from a Third World dump ... which meant half the world was probably listening in on it.