I knew a guy who was investigated by the Secret Service for having his picture taken by a friend in the rear area of a federal courthouse, of which neither of them were aware.
The agents quickly discerned that he was no threat, but then also made it known that they were required both to ask him provocative questions *and* that they had to talk to him for a minimum amount of time, about an hour, even if nobody had anything pertinent to talk about.
He said the provocative questions obviously were designed to provoke an emotional response that would set a kook off. The rest of the interview was intentionally dull, an effort to get the subject to blather in an unguarded manner.
I was similarly interviewed by Federal Marshals.
Was in a hotel lounge in Manhattan waiting (for hours) for someone. Got bored and started fiddling with my video camera, and recorded a couple interesting cars going by. Long story short, soon thereafter two Marshals showed up on “reports of suspicious activity”, yakked with me for a while, asked a couple speed-bump questions (not hostile, but jarring to the conversation’s flow), looked at the video, and eventually left - commenting that BTW, there was nobody of note in that car. They never asked for ID.
Next day I discovered President Bush was giving a speech that morning at a nearby hotel.