At what point are they considered to have quit? They can’t just not show up for work.
You're confusing political-world for the real world. In any other job, if you're gone for several days without notice, you've abandoned your job. In the political world, you're not gone until you either resign, or are expelled.
They probably can't expel anyone without a quorum.
“At what point are they considered to have quit? They can’t just not show up for work.”
I’m wondering too. You have MILLIONS of people in Wisconsin right now without representation, and no sign of those Senators. I hate to think horrible thoughts (LOL), but if a Senator was kidnapped or just disappeared, there must be a way to declare the seat vacant have a special election. Why not do it for all of them? And, in the meantime, with the seats vacant, the number to form a quorum drops and the Senators that were lucky enough to have avoided such a demise (LOL) can get back to doing the work of the people.