If extinguishers are there for staff use only and if staff are advised as to its unique location is there an issue really?
I know what you're saying NT, but the requirement to be clearly visible or signed is there for a reason. My view on it is that in a panicked state looking for an extinguisher you may forget about your training as to where this specific extinguisher is and just not see it. There's a lot of depends isn't there? Level of training, size of building, number of staff, frequency of staffing changes, temporary staff etc. Plus, I can sell them a big sign! Let alone the fact that it's a powder extinguisher indoors - much better to make it more obvious by changing for a water spray and a CO2 (and mounting them lower).