lita-way is a lita.io handler that saves 'away from keyboard' status messages and informs other users via DM if they @mention a user who is currently afk. Currently very beta. Only known to (sort of) work in Slack.
Add lita-away to your Lita instance's Gemfile:
gem "lita-away"
Tell your bot you are going to be afk:
@mybot afk Going to grab a burrito
Now when anyone mentions you directly, your bot will send a DM to unobtrusively let them know what you are up to. The mention will have to be preceded by an "@" or it will not register.
When you get back from your errand, delete your status message with @mybot afk-undo
.