I could use some good terms for security clearances in 40k. Rather than saying an Inquisitor has "Inquisitorial level clearance" it would be nice to have something more flavorful to use, same for acolytes, Arbites, local law enforcement, etc. Unfortunately I haven't been able to come up with much ... any help?

Back in the day I started with "The Judicious Remit of the Acolyte Inquisitor" wherein acolytes are discerned a direct portion of well-defined authority and access.

In-universe, 'Dark Magenta' was once hailed as the ultimate coding sigil of the Inquisition, though Zou's book revised this and Abnett introduced the 'going blue' of an Inquisitor going properly 'undercover', even with regards to his peers.

That said, I'm not terribly familiar with modern 'security levels' beyond that used at uni. (Student access, public access, technician access, janitorial access [pretty much everything but only given to folks who're checked and not likely to interfere with stuff], cleaners access, catering access, professorial access [to lecture theatres and your own labs] etc)

Terming that right for a translation would require a sensible structure to map onto. I can't give you a more helpful one offhand, but noting down the structure of wherever you work (be it federal government or a fast food van) or regularly visit should give you a quick idea for the structure you'd need, with the remainder merely being a case of fitting the correct archaic/cool/pseudolatin names to their realworld counterparts.

Except Auditors. Auditors and HMI Inspectors get remarkable access, being the sort of official/bureacratic analogy to Inquisitors.

With that in mind, many of my characters are former auditors. Either for local things or for more grand services (the Adeptus Terra's Audit Imperialis? In the grim darkness of the far future, there is only proper bookkeeping...)

With all that in mind, getting the correct 'feel' for the access levels might help alot. And keeping that feel to be more 'mundane' than 'cooool!' is probably helpful for the players. Making them think they're sneaking in as janitors or network technicians or plumbers might help them feel a bit more comfortable in RPing subtly than, say, simply marching in and saying "We're the Inquisition, do as we say!"

I think it was the IH that mentioned the "Letter of Investigation": this is an authorisation (most often a real letter with some ultra-modern identification/verification system attached to it so you can prove that you "are who you are") given by an Inquisitor to his acolytes. It states what they are investigating and that they are allowed (by Imperial Laws, I supposed) to put to question people that might have connection to [MatterOfInvestigation] and that they are allowed to search place that might have connection to [MatterOfInvestigation].

In my games , this normally broils down to the point that the higher up they try to enforce entry/questioning (which goes beyound a polite Q&A session or a quick look around at the premisses) the more likely it becomes that some sort of "Legal Adept" will ask how his master/his masters premissis are supposed to be attached to the [MOI].

The more vague the acolythes answer (" because everyone is supsect and innocence proofes nothing! " being the ultimate in wibbly-wobbly) the more likely the "Legal Adept" is to make a legal case of this matter... (unless the acolytes pull of a good intimidation test!). His master might not be alowed to leave till a Judge (or what ever passes as this!) has made decisions about this, but the acolytes are not allowed to bother him and so the Inquiery comes to an anoying stall. Skill ins "Buerocrazy" and/or "Scholastic Lore: Law" (do not remeber the name) will help to speed this along/make the outcome vavourable to the pc.
Of course, good answers and cues will make legal hazzling less likely (you think twice before going to court against HENCHMEN of the =I=...since it might just turn the real attention of the =I= on you...which makes since worse and beyound court!).

Of course, the pc still can try to do there things "covered" and trust on the "Letter" as a "going out of jail" card if they get caught.

Talking Arbitrators:
I simply made the player of our Arbitrator make roles for the "Aribtes" Loreskill when ever he need a "legal reason" to inspect something or some-one under "imperial law". The difficulty always took into account what clever ideas for the "why?" the player came up with and if/how much the game benefitted from it.

Given the way the Inquisition classifies threats (xenos minoris, hereticus majoris, etc.), I wouldn't be at all surprised if they used some sort of Latin-ish system. For example:

Confidentia Minima (appropriate for rookie acolytes)

Confidentia Minoris (appropriate for proven acolytes)

Confidentia Majoris (appropriate for trusted acolytes)

Confidentia Extremis (appropriate for Inquisitors)

Confidentia Terminus (appropriate for Inquisitor Lords )

Perhaps, in keeping with the ultra-paranoid aspect of the Inquisition, the above mentioned naming conventions could outright mimic the designations for enemies in DH?

Ie, the security ("confidentia") level is based at least partly on what sort of 'damage' an agent could do if they were to go rogue, with acolytes at the lower end (they could foil a few direct plans or act as mooks for someone else), while Inquisitor Lords have such authority (word travels slow across the Imperium) and allies, that in fitting with "Terminus" ratings, they could destabilise an entire sector and turn it to their own ends given half the chance, and should be eliminated with both extreme prejudice and the very best tools for the job.

The old Inquisitor comics, back when Warhammer Monthly was still around, were from the point of view of an Inquisitor's senior Acolyte. One thing suggested was that the Acolyte's clearance seemed essentially unlimited, but his access was restricted to whatever ciphers and codes he could decode himself. I quite like the idea, as it allows for an Inquisitor to see who amongst his servants are right for the job - those with the will and the wit to find the information they need for their missions are those to be watched carefully either as future Inquisitors or as dangers to the Imperium.