Normal commute to DC, probably. Normal commute to the chambers of the Senate, probably not.
It is not enough to be on the way back to DC, or even on the way back to Senate Offices. The protection exists only so long as a Member of Congress is on the way to, at, or coming from a session of Congress. That was the original intent. As I (and wiki, for what that's worth) read it, a member of Congress ( and his staff) is protected from arrest while going to and from sessions unless he is suspected of having committed a felony or treason, and is protected from civil or criminal actions stemming from speeches or debates while in session.
While refusing to obey the illegal orders of the TSA would not constitute a felony, I believe the orders of the TSA to have been legal in this case, and Paul's refusal to obey is at least technically a felony and immediately stripped hi ofnthe protections (that he didn't have because he was not heading to a Session of Congress).