We recently had a hiccup in which our nightly builds were stalled due to someone or something placing the master in Prepare for Shutdown mode. But with nothing logged for this request, we had no clues to go on. It would be handy to have these requests logged.

Unless I'm misreading the source, it looks like jenkins.model.Jenkins#doQuietDown just needs a logger call. Likewise with #doCancelQuietDown. Something along the lines of the following.

Putting Jenkins in quiet mode as requested by <someUser>
Canceling quiet mode as requested by <someUser>

(Note: We ruled out ThinBackup, which does log when it forces quiet mode. We don't have TB configured to force quiet mode.)