I'm not sure there any formally mandated consequences for not attending. On the other hand:
Here are the guidelines for how senators are to conduct themselves during the impeachment trial of President Donald Trump, which is expected to begin Tuesday. They were put out by Senate Majority Leader Mitch McConnell, R-Ky.
- Senators should plan to be in attendance at all times during the proceedings.
- Reading materials should be confined to only those readings which pertain to the matter before the Senate.
So the guidelines are that they should all be present and pay attention to the proceedings.
Interestingly, an article on the Heritage Foundation site mentions a related issue. [Senate] Rule XI, which
allows the appointment of a small number of Senators to operate as a trial committee to gather evidence and take testimony. The Senate has used trial committees on only three occasions in the 1980s to assist with fact-finding regarding impeachment articles approved by the House against three federal district judges. [...]
Rule XI was adopted as a response to poor attendance and preparation by Senators in impeachment trials in the early twentieth century. Yet even in the 1980s, some Senators claimed that they had not bothered to prepare before voting, and such proceedings diverted their energies away from legislative business of greater concern to their constituents.
Another article (alas in a tabloidish pro-Trump source) mentions that
“The senate collectively has the power to compel the attendance of absent senators and the Senate collectively acts by its majority and Mitch McConnell is the majority leader. So he has the ability,” former Senate parliamentarian Alan Frumin said. “If push comes to shove, compelling means arresting. ”
(N.B. a "Senate parliamentarian" is an "official advisor to the United States Senate on the interpretation of Standing Rules of the United States Senate and parliamentary procedure.")
The article in question does not mention any precedents of this kind of compelling actually happening though. Actually, it later says:
Frumin, who served on the Senate floor as senior assistant parliamentarian during the Clinton trial, said there was no expectation at the time that senators needed to be present for every second of the event and he recalled regular attendance was “sparse.”
“They came and went. If you are envisioning a situation where during the trial, 100 senators are sitting at their desks in rapt attention — that didn’t happen,” he said.