You can manage the settings of the Kiosk to specify that if a volunteer previously requested a substitute, if that substitute request had not been filled and there was no one able to take their place during sign-in, to not flag them as a "no-show" on the schedule.
1) Go to the Web Terminal pane in MSP.
2) Click the "Web Terminal Settings..." button at the top of the Home tab.
3) In the Settings window, click on Sign-in Kiosk on the left.
4) In this set of options, check the box for, "Do not flag volunteers who have requested a swap as "No Shows."
Click OK in this window and then click the Commit Changes button at the bottom of the Web Terminal pane.