Perhaps not so easy. Article 2, Section 6 of the Constitution reads...
They shall in all Cases, except Treason, Felony and Breach of the Peace, 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.
For discussion.
The legislative bodies can place members outside of those protections, as Rostenkowski & Traficant discovered.
And all this is saying is that they cannot be arrexted while Congress is in session, it says nothing about afterward. And in any case, that is irrelevant, leaking classified information is a felony and they can be arrested for this at any time.
As far as not being questioned in "any other Place," that only applies if the offense occurs during a speech or debate in Congress, this offense didn't.