The quickest way to do this with no additional customization would be to create a new group… you could call it “out of office” or whatever you need. The group can have a custom flair icon and user title… so you could do something like this:
Then when someone is gone they can join that group, and when they’re back, they can leave it.
It’s also worth mentioning that the Discourse Calendar plugin has an availability calendar feature that can do something similar, with the bonus of doing it automatically based on a set date range, but at the moment when someone is away it’s only visible to admins and moderators.