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.
Each house shall be the judge of the elections, returns and qualifications of its own members; and a majority of each shall constitute a quorum to do business, but a smaller number may adjourn from day to day, and may compel the attendance of absent members in such manner and under such penalties as each house may provide.
Article VIII, Section 8, Wisconsin Constitution:
On the passage in either house of the legislature of any law which imposes, continues or renews a tax, or creates a debt or charge, or makes, continues or renews an appropriation of public or trust money, or releases, discharges or commutes a claim or demand of the state, the question shall be taken by yeas and nays, which shall be duly entered on the journal; and three-fifths of all the members elected to such house shall in all such cases be required to constitute a quorum therein.
Regrettably, Article IV, Section 8 requires a two-thirds vote of all members elected to expel a member. I would think that suspending the pay of any absent member would be permitted under IV.7. Potentially they could get away with hiring a bounty hunter and upon not finding the person, declare the seat vacant.