I logged into one of the sites I moderate this morning and just happened to check the dashboard and noticed that one of the mods had recorded their absence. It got me thinking--if I hadn't happened to check, I would have had no idea we were down one mod. Now, I know any mod can easily notify fellow mods in chat, but I understand how sometimes that might fall by the wayside (it certainly has for me before). The mod absence record is already completely visible to other mods, so would it be possible to have a checkbox above it to be notified of changes? I don't care whether it's a Stack Exchange inbox notification or an email or either/both.
The least intrusive solution might be to put an indication on the main dashboard at
/admin/dashboard where the outstanding flags are displayed as this is probably where most moderators go to see if there's anything that needs their immediate attention.
This doesn't require you to view any other pages or opt in to a notification to find out that a moderator has marked themselves as absent and most moderators will visit the dashboard on a regular basis.