Not much cover for a nine foot tall genetically modified super soldier.
Sorry... rather pointless post of mine.
Or things have gone so incredibly badly that the Inquisitor and his team are completely screwed unless the Deathwatch wade in and begin slaughtering a way back out.
If I recall, something very much like this happened somewhere in a novel. Not necessarily 40K, but somewhere... At least once.
"Truth is so precious it must be attended by a bodyguard of lies."
"They say that once you have opened the final gate there is no way back."
I know that at least in Eisenhorn, once the "True Matter" of the Necroteuch was out in the open was only when the Deathwatch stepped in.
Faith is an armour harder than any metal, but more delicate than any flower.
Sorry for the big post, a lot of thoughts to get out.
Taken from http://wh40k.lexicanum.com/wiki/Deathwatch
"The orders of the Deathwatch are not merely the cleansing of xenos cultures. They also include the recovery and study of alien devices and artifacts. Sometimes it is necessary to use a weapon against the enemy who created it, although this is not taken lightly. The Deathwatch are constantly vigilant for sabotage, or to advise if it is truly safe to use a weapon of xenos origin. The Adeptus Mechanicus are always on the lookout for alien technology; the C'tan Phase Sword, used by the Callidus assassin was recovered from a Necron tomb world and successfully integrated into the arsenal of the Imperium.
Usually, a Deathwatch team is led by an Inquisitor, but in extreme circumstances, a Deathwatch Captain or Librarian may take command of the unit. Their word is law, and can requisition anything they desire to get the job done. They are the best armed and trained units in the Imperium. They might be deployed in areas where conventional methods are insufficient or if the troops available are not properly equipped or trained for the task. The presence of a Deathwatch team is always welcomed, even though they are feared by all those who know of them. "
As you see there is a bit more than just see xeno's kill xeno's. Yes there will be alot of combat but that is probably one of the purposes of this book, more indepth (and survivable) combat. Very rough grouping of the themes of the books (all just my opinion). DH=Investigate/Infiltrate/Purge RT=Explore/Conquor/Profit DW=Plan/Study/Purge.
There won't be any 'undercover' work but they will still need to deal with the local's, organizing defenses, gaining the local knowledge of the land, possibly needing to put down any resistance to their presence which would be rare but if under the command of an Inquisitor just killing off local's might be frowned upon.
Then there's the planning, scouting the area, learning the possitions and weaknesses of the threat. The Deathwatch is the brightest and best of a variety of chapters, they aren't going to just always go in bolter's blazing when they might be outmatched for a frontal assault. They are also smart enough to know such methods might allow some xeno's to escape which would be unacceptable. As in the quote from above 'when conventional methods are insufficient' conventional methods in the imperium are usually throwing bodies at a threat until it is gone....so they are usually called when mass assult (and thus a pure maim kill burn method) is rulled out.
And study, they are not the best at killing xeno's just because they have killed more than others. They also spend alot of time studying xeno's and their tech, and if they themselves are not doing the studying most specimens to be studied must be aquired in the field.
Example: Inactive Necron tomb world found, Inquisitor requisitions a squad of Deathwatch to assist him since if there is a fight they will be the best to have around. The inquisitor is not going to be doing all the searching, so the DW will have to help with many tasks. But when they arrive a AdMech Mago's and his team have beat the Inquisitor there. The Inquisitor (assuming he's an NPC and not a player in the group as will be an option by the time DW comes out) leaves it up to the DW to find a solution to this. Do they make it a who has the bigger penis match and push the AdMech out? Do they find a way that both sides can work together for the benifit of the imperium? Then parts of the tomb awaken...while they are all deep inside, even the lowest INT marine knows their small force can't take on a whole tomb world by itself soo....Do they try to make a stealthy retreat out, form a defensive possition and weather a seige inside the tomb until reinforcements arrive, or work with (the hopefully still around) AdMech group to find a way to disable/destroy the tomb world before it can fully awaken to become a bigger threat. Plus little about necron's is known, with whatever option they take will they also attempt to retrieve any tech or data that may be studied to better help the Imperium fight them, or possibly just keep the knowledge within their Deathwatch chapter.
The players 'could' just try to bolt everything down....but then as a GM I would probably have to let them die...and they tell them to reroll real Deathwatch marines, ones that are smart enough to know that a bolt pistol is not the solution to All problems. The Deathwatch is the best of the best at dealing with xenos, They aren't going to be much better at Killing xeno's then normal space marines, they are the best with Dealing with them. And Dealing with can be many many more things than killing them, thats just the easiest method so the one the 'non-elites' will think of first.