Why not knocking is the right default
In a consulting room, a meeting room or a private office, an interruption is at best inconvenient and at worst a genuine problem. The cleaner has no way of knowing what is happening inside, and a knock puts the occupants in the position of having to respond. Moving on quietly costs the cleaner a few minutes and costs the occupants nothing, which is the correct balance. Moving on quietly is the response that costs the least to everyone involved.
The exception is where a client has specifically asked for a different arrangement, such as a knock in a general office where staff are happy to be asked. That is recorded in the site instruction rather than left to individual judgement, because a cleaner applying a rule from one site to another produces exactly the awkward situation both parties wanted to avoid. Rules that travel between sites unexamined are how avoidable awkwardness is created.
