So, I homebrewed a bit that before the PCs entered a Manse they would have to relinquish their weapons and a servo skull would scan for said weapons. However, one of the PCs had a concealed holster and asked if the servo skull would detect his laspistol. It didnt matter to me if the PC brought in the laspistol or not, but how should the opposition check of the servo skulls scanning vs concealed weapon play out?

What is the method the servo-skull uses for "scanning"?

A skull that is the equivalent of a hand-held metal detector & the weapon is detectabel this way will likely find said item.

A skull that is equivalent of the "body scanners" that are in use at some airports nowadays will even "register" a weapon that is not detectable by a metal detector. The concealled holster will not help here, either.

But, both of the methods mentioned above aren´t "fool proof". I would still call for a check (on behalf of the servo skull) and take the size of the weapon into account.

Generally, a "concealled holster" helps against eyes and "hands on". It is meant to make the weapon fit to your body in a way that makes it harder to note. If any kind of technique is used that includes "rays" to detect something (and thereby relies on the interpretation of the way how or if those rays are reflected back from the scanned body) the holster will only help if the material it is made of was designed to fend such thing off. Which is "GM´s choice" if it was not talked about it before.

The third method of detection, by the by, is "passive scanning for emission". A dog sniffing for drugs is a biological emission scanning device (scan for certain charateristic ordors emitted by drugs). A "heat scanner" is a technical example, it registers heat that is emitted and MIGHT register things on your body as "strange blotches" if they block the heat your body emmits in a different way then your clothes are doing.

I hope this was helpful.

RAW, the concealed holster grants -20 to detect the weapon. And I believe attempts to detect hidden things are opposed rolls, Stealth vs Awareness (although feel free to change this up depending on the situation). So figure out what the skull's awareness is and do the opposed roll.

I would think that any concealed holster manufactured in the 41st millennium would be made to counter all of the forms of detection Greg mentions, plus a lot of the ones we don't have technology to do.

I would say either have it as an opposed test of sleight of hand vs. awareness giving the skull the auspex bonus but penalty for the concealed holster or no opposed test instead have the skull make a tech use test to detect non-visible objects with the auspex no benefit from the concealed holster.

When it comes to 40k, i assume everything is over-engineered junk. This in mind, that concealed holster is more than just a shoulder strap of today. Maybe it is lined with some sort of dampening material that prevents detection. Either way, if you PC thought to go out of his way to get a concealed holster, i would say let him take the opposed test to show that their foresight has value. After all, whats the point of going out of your way to prepare for a specific situation if your preparation will have no effect.

I ended up taking an opposed test Stealth vs Awareness. Since this is my first GMing experience and the PCs are newish to the 40K world, I am kinda placing NPCs or events into the "Dark Pursuits" adventure that do not have major consequences and this allows me to experience the mechanics and the PCs to get a feel for the 40K universe.

Yeah stealth awareness. I would give the player +20 for the holster, and +20 for size. Then roll against the skull that may or may not have a +20 if it's specially equipped to scan for weapons.

Is it really necessary to make an opposed test? I just left home so I don't have my book but if the holster makes it harder to detect a weapon just make a appropriate test (challenging?) for the skull and make that roll -20.

That's at least how I would do it while minimizing the rolls at the table.

Edit:
Now that I'm at home and have my book I still believe that a lone Awareness test should be the most appropriate. Stealth is presented as a skill to hide yourself and not an item. While I don't know how the rules handle hiding stuff I'd say it's a Awareness test with modifications depending on if somebody has hidden the object you're searching for.

Honestly Hikage, that is the quickest way to do it and totally appropriate. It's honestly how I've done it more than once if I wasn't expecting a pat down, and wanted to keep the rolls light. It's a perfectly fine way to do it, and I imagine it's how most searches have been handled by most GMs. That said I think that the opposed rolls give stealthly characters another way to excel, and in general will mostly favor badass PCs rolling under 70 after bonuses vs some oaf in a nice suit rolling under 35. So I like that ruling, I like both rulings, and I've ruled it both ways. As far as what the rulebook says on the subject when looking at my Only War book I found on page 132

Now, however you choose to interpret that: "Does it mean palming a knife from under a pillow when the guards come into your cell and are looking at you, or does it mean slipping a needle pistol in a special holster into the Governor's Palace past the checkpoint?" To me matters less than what you think is more fun for your table. And a single awareness roll is a very simple fast way to be done with it.

These are the rules I would use.

Pg 115 Sleight of Hand

"Sleight of Hand also allows a character to conceal his actions while in plain sight, such as hiding weapons from someone searching his person."

Pg 100 Awareness

"Awareness is used when trying to spot items or individuals that are either hidden or not easy to notice"

Pg 138 table 4-6 Size

"Puny (2) Bolt pistol +20 Stealth"

Pg 170 Concealed Holster

"Attempts to detect such a weapon suffer a -20 penalty"

Pg 177 Monotask Servo-Skull - Augur

"The character gains the benefits of an auspex"

Pg 175 Auspex

"A character using an auspex gains a +20 bonus to Awareness tests"

Awareness(+0) Overall the Holster grants a -20 to the awareness test, the Auspex grants a +20 so there it is no modifier in the end.

vs

Sleight of Hand(+20) the las pistol is a puny weapon (smaller than a bolt pistol but not small enough for a knife) so easy to conceal.

Servo Skulls are in the rogue trader book pg 375 Per 35 Awareness +10 however rogue trader is far higher power level than Dark Heresy so drop it to 30 perception, awareness rank 1. I like the argument to use stealth, but i think where possible it is best to avoid making the skills that are already excellent like stealth, even better by allowing them to do things that other less useful skills do. Using the size modifications is nessecary to counter out the many easily gained awareness bonuses, and also, to show players the difference between trying to hide a pistol and an autogun.

That's the bonuses I would use, and using slight of hand over stealth is fair enough. In the hundreds of hours I've either played or run games with these rules. I can't recall making any sleight of hand tests, I we would always forget it existed in the moment and used stealth instead, even if it was something that explicitly should have been sleight of hand. So that's not a bad idea at all.