The inestimable Jet-Poop is perfectly correct; no-one has yet noded the term janitor. We just can't have that. Allow me, as a member of the noble profession in question, to elucidate upon the terminology proffered herewith.
Various sources tell us that the word janitor is derived (at base) from the Latin term iænus, which means doorway (or gateway, covered passage, portal). The proper form IÆanus was anglicized to Janus - the God of Doorways, or the Gatekeeper. The term iænitus denoted the 'keeper' of a door or passage; the American Heritage Dictionary tells us that Saint Peter was referred to as the 'Janitor of Heaven' in his role as doorkeeper. It further notes that the first anglic use of the term is probably from a Scots text circa 1567. Eventually, the 'keeper of the keys' came to mean the keeper of an establishment; eventually, the maintenance functions overtook the ownership and mastery connotations, leaving us with our humble but crucial janitor of today.
A custodian, of course, is another term for the janitor.
We are they who toil late
when all the lights burn low
who seek to learn what men would hide in desks
and deny us the right to know
Pushing brooms and mops and wielding lights
of brushed aluminum and steel
we walk the darkened corridors
in guard of all that's real
for otherworldly danger lurks
in shadows grown long across the floor
from flick'ring candles, lowered lamps
or moonlight shining round the door
Monsters, villains, miscreants young and old
Who would seek to invade our halls
'tis our noble duty to them deter
or else find them, and kick them in the balls.
Truly, a noble profession.