It is confusing because the SC has struggled so much with the issue in the past. They used to analyze the 4th A cases with a property analysis, and whether a trespass was committed by the govt. But since the famous katz case, it uses a privacy analysis. That said, where one is does affect the expectation of privacy.
Let me give you the extremes. MOST 4TH A PROTECTION: Man tells wife (whispering under the covers (alone)) that he robbed the mini-mart last week. The police recorded it with a listening device they planted in his bedroom just because they don't like him. No evidence or reason to believe he committed the crime. OK, the exclusionary rule keeps out this evidence. The was no warrant, and no exigent circumstances permitted this surveillance (for a criminal investigation).
The other side of the scale. Man with heavy overcoat and sweatpants enters the Capitol building in July with a turban on his head, and a backpack two feet deep on his back. Police search the bag and find a nuke. No 4th A problem here. There was no expectation of privacy when entering the Capitol building. The search was therefore not a "search", thus no warrant was required.
Clear? Ya, me neither. OK, back to work.
That does sound complicated...but which category should talking to a terrorist fall under?