If those are taken, then there are synonyms like sentry, sentinel, guard, guardian, watchman, protector, monitor, screener, defender, shield, chaperone, paladin, vigilante, ...

How did they end up with "janitor". I'm looking at thesaurus.com and it doesn't list "janitor" as a synonym for any word that denotes a protector of something.

There is "custodian"; sometimes building janitors are called custodians, but that word has connotations of taking care of something (having it in your custody).