Sen. Rand Paul ‘detained’ by TSA in Nashville, TSA denies [More]
It doesn't matter what they call it, in this case, he may really be an exempt "Only One" on the basis of Constitutionally-required separation of powers:
The Senators and Representatives shall ...be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
I'm not a lawyer, but it will be interesting to see if this is an issue. Reps have made such claims for much more far-fetched reasons...