Youtube edutainer CGPGrey posted this video explaining at the linked timestamp, a pro-forma session of the Senate.
He explains that the Senate can "take a recess" without officially doing so, by holding a pro-forma session in which one senator announces a 3 day break for the Senate, doing so on every 4th day as long as the Senate wants to vacation.
Of interest is that, in the video, Grey says that this can only be done because the Senate always assumes a quorum, unless "someone asks for a roll call" to show otherwise.
This document says on page 5 under "Ordering the Yeas and Nays," however, that one-fifth of a quorum must ask for a roll call.
Does that mean, then, that, barring a constitutional amendment, it is not possible to prevent the Senate from using this recess loophole?