There has to be a quorum in the chamber for the Senate to be in session.  I don't think there has to be a quorum in the chamber to pass legislation.  I know a Quorum Call is a standard tactic when someone wants to be a PITA, for some reason, and drag everybody from whatever it was they were doing into the chamber to prove there is a quorum "present."  

So, I'd say as long as nobody questioned it and under Unanimous Consent it was passed under the Rules of the Senate.

Ever since I learnt about confirmation bias I've started seeing it everywhere