Yes...a die-roll mechanic is what I was looking for, though an "Awareness check" seems to create surprise all too much. Most beginning PC's have an Awareness score in the 30's....which means they are surprised about two-thirds of the time. Does anybody who has the rulebook care to clarify this?
The thing with Dark Heresy (and Rogue Trader as well, based on the quick-start rules) is that quite a lot of play is spent putting yourself in a position to gain extra modifiers to your base stats. For example a character with BS 30 is a pretty poor shot until he aims as a full-round action then upens up on full-auto.
Similarly, there will be circumstances that increase and decrease awareness: In the aforementioned example of guards behind a door; are the guards paying attention to the door or playing cards? Are the assaulting group sneaking up to the door then attempting to determine what the guards are up to, or clattering down a metal walkway at full tilt?