Several of my characters have cleverly acquired nice armor and weapons, which is great. However, they're about to start some scenarios (e.g., Purge the Unclean) where they will be attending parties with nobility, religious ceremonies, traveling to under-hives, etc.

Can they keep their cover (e.g., "a bodyguard") while carrying around a giant lasgun backpack and ceramite armor? Is this normal attire for a underhive gang meeting or a party?

My noble assassin has it easy, as a moritat he's happy with just his sanctified mono sabre and xenos mesh armour under his clothing.

I think a character should get around the fact he can't take everything he owns to every occassion, its lazy and makes the gear more important than the persons skills and character.

If something should happen at a social occasion where they are under-equipped its more of an accomplishment to get out alive by using your wits and ability to turn the situation to your advantage in some way. Far better than simply shrugging of the damage and inflicting massive damage back.

Every situation should be different though, the social etiquette of attending a hiver gang meeting would warrant being tooled up and it would be stupid not too.

Several of my characters have cleverly acquired nice armor and weapons, which is great. However, they're about to start some scenarios (e.g., Purge the Unclean) where they will be attending parties with nobility, religious ceremonies, traveling to under-hives, etc.

Can they keep their cover (e.g., "a bodyguard") while carrying around a giant lasgun backpack and ceramite armor? Is this normal attire for a underhive gang meeting or a party?

Not really..

part of the fun in DH is dressing so your character blends with the locals.

Even for the under hive gangers, the PC with giant lasgun backpack and ceramite armor, would be prime target if things go bad. Where as Brother Anselm's Noble Moritat may not be looked at as a prime threat until it is to late, just armed with saber.

Several of my characters have cleverly acquired nice armor and weapons, which is great. However, they're about to start some scenarios (e.g., Purge the Unclean) where they will be attending parties with nobility, religious ceremonies, traveling to under-hives, etc.

Can they keep their cover (e.g., "a bodyguard") while carrying around a giant lasgun backpack and ceramite armor? Is this normal attire for a underhive gang meeting or a party?

If their cover is "bodyguard", then I would say, sure, a noble might have a heavily armoured bodyguard with him at a party - dressed appropriately, of course, so make sure that armour and gun is gleaming, and shave properly. But yes, I would go along with that under the right circumstances (although as a bodyguard, at a closed nobilite party, they might have to guard the entrance or may not be permitted to carry weapons; it might be better to go as the noble's +1, and dress for the role).

As for a meeting in the underhives.. I would say no. Anyone with such high-tier weapons and armour and looking like a military professional would raise suspicion. Maybe if it is readily apparent that you are playing some kind of high-tier crime boss and that this guy clad out in full armour and wielding what is essentially a low-powered lascannon is the bodyguard.. maybe.

But just walking around down there like that would be weird to the local populace. Either everyone you want to talk to will bolt out of sight and out of mind, or you'll be attacked by people wanting to either rob you or simply take you out for the threat you might represent.

The effects of dressing appropriately (or not) should be roleplayed, and you shouldn't auto-penalize your players for breaking this, but you might want to make it clear to them that they're not "doing it right"; but on the other hand, they might want to go down into the Underhive and bury themselves in heaps of subhuman filth, blood and guts.

I've played purge the unclean and the "rules" for weapons are stated clearly. I think bodyguards could have simple sidearms etc, not run around with heavy stuff.

As for the party, I'd presume armed men wouldn't be allowed into an invite only place that's heavily guarded already. Makes those compact weapons all the more useful.

My players invariably act surprised when I point out that a group of people wearing battle-scarred carapace armour and bristling with weapons (including a flamer, missile launcher and/or heavy stubber) attract a lot of attention- and they flatly refuse to stow their gear during investigations, insisting that it is sufficient to just 'act casual'...

My players invariably act surprised when I point out that a group of people wearing battle-scarred carapace armour and bristling with weapons (including a flamer, missile launcher and/or heavy stubber) attract a lot of attention- and they flatly refuse to stow their gear during investigations, insisting that it is sufficient to just 'act casual'...

Of course the locals are in no way put off by such impressive firepower being casually waved around in their faces.

I for one find a person thinks a flamer is casual wear to have a horrifically skewed sense of normality.

It is crude and inelegant but sometimes a hard reset of gear is justfied. In such circumstances it cushions the blow to allow the layers to choose a few thing to keep (I allow two weapons, one clothing and one armour.) It could be acheived in a number of ways the easiest (laziest) of which is direct orders grom the boss with issued gear.

for example.

Quietly infiltrate this mining guild`s strike meeting and extract the leader of the guild for questioning, leave no trace. Here are your disguises and compact laspistols.

I've played purge the unclean and the "rules" for weapons are stated clearly. I think bodyguards could have simple sidearms etc, not run around with heavy stuff.

The Archmandrite's Ordination in Ascension has similar restrictions; sidearms only, worn openly (so you can see who's armed), all bound up with a symbolic ribbon.

You'll find things like that in quite a lot of environments; having to deal with it is one of the big differences between Dark Heresy and Deathwatch. Even a Vindicere becomes a lot more threaten-able when armed only with his pistol and forced to wear normal clothing rather than a cylon gimp suit.