One who cleans for a (usually pretty poor) living. This job is, quite literally, as old as dirt - or, more accurately, as old as people being able to get someone else to clean their dirt up for them. Normally this is the epitome of low skill, low wage labour, frequently in the black economy or grey economy - although in some places even the cleaners are highly skilled and well paid1. If a cleaner is employed by the people he or she cleans for then they're normally called a janitor (or a maid/valet/footman/servant2 in a domestic setting) but in the modern world cleaners are generally contracted by an outside company. In a hotel we call them "housekeeping" and on a ship "stewards".
Now, the thing about cleaners is that all too often they're practically invisible and get treated like ambulatory furniture, which makes them the indoor equivalent of beggars in terms of urban camouflage. This means that 'cleaner' is a great disguise for someone looking to infiltrate a modern corporation:
For a start, people will (often deliberately) ignore cleaners, and even if they don't, these tend to be jobs held down by immigrants with poor local language skills so pretending incomprehension when questioned is pretty easy. Even when they are noticed, the frequently high turnover of staff in these jobs means that a new face can go unremarked. Also, cleaner get everywhere in a company - at the right time of day, no-one will bat an eyelid at a cleaner going into the CEO's office or the CCTV control room. The server room might be more of a challenge but not much.
Then, the cleaner's trolley or mop bucket can conceal pretty much as much equipment as you need for the job (and any loot - or people - you are removing) and if you come to a door you cannot open you simply clean around it until someone comes through, and then you prop it open and tailgate them.
The other cleaners might notice you, but they rarely have a vested interest in the security of the place they are cleaning, and if their immigration status is dodgy (as it often is) they won't want to see the police anyway.
Cleaners often work well as an 'invisible witness' as well - the smart investigator can deduce that there was a cleaner present, whom the criminal ignored, but the cleaner has gone to ground (perhaps because he is an illegal immigrant) and at least half the mystery is tracking him down.
A very specific form of cleaner, best known in British fiction, is the Char Lady - a part time domestic help, usually a middle aged or older woman - who cleans house and undertakes other menial domestic duties. They are traditionally a mine of information for anyone prepared to deal with their rambling conversational style and forthright opinions … and their tendency to make themselves scarce when there is trouble in the wind.
Not to be confused with the Evidence Elimination Expert … who is often called a cleaner.