I believe that SoM can only show build errors reported by the builders in question. In this case, the build machine was fully offline, and so wasn't generating build errors.

In theory, master builders could (do?) report slave builders that never start. 

The more robust solution would be to have som-dispatcher query and part buildbucket requests to look for builders that timeout, or fail to start before any CIDB entries are created, or after the report stage has finished writing to CIDB.

Of course, that would require more development work.

Yeah SoM uses CIDB as a basis to find slave builds.  It does query Milo for some additional information, but only for builds it find via CIDB.