The special ability invisibility offers some guidance…
It's unfortunate that in the Player's Handbook neither the skill Listen nor the skill Move Silently addresses what happens when a creature is standing still or performing an action that involves moving but that doesn't involve traveling from square to square using its speed. However, the Dungeon Master's Guide on Invisibility does indirectly address this possibility:
A creature can use hearing to find an invisible creature. A character can make a Listen check for this purpose as a free action each round. A Listen check result at least equal to the invisible creature’s Move Silently check result reveals its presence. (A creature with no ranks in Move Silently makes a Move Silently check as a Dexterity check to which an armor check penalty applies.) A successful check lets a character hear an invisible creature "over there somewhere." It’s practically impossible to pinpoint the location of an invisible creature. A Listen check that beats the DC by 20 pinpoints the invisible creature’s location. (295)
(Emphases mine.) By extension, then, a Listen skill check result that at least equals a visible creature's Move Silently skill check result should likewise reveal a visible creature's presence! (The Listen skill check DC should, of course, be adjusted for distance and barriers and so on as normal, obviously.)
That is, in the typical campaign, a creature can only take free actions (except speaking) on its turn. This means that, off-turn, when the standing-still creature's not doing anything, that standing-still creature nonetheless makes a Move Silently skill check if the foe attempts to discern the standing-still creature's presence. (This, by the way, utterly ignores the idea of combat simultaneity and what actions the standing-still creature took on its turn!)
Then, if the foe's Listen skill check equals or exceeds the creature's Move Silently skill check, the foe determines that the standing-still creature's over there somewhere. And if the foe's Listen skill check equals or exceeds the creature's Move Silently skill check by at least 20, the foe pinpoints that the standing-still creature's location.
But that's just for using the Listen skill in isolation. It's usually pretty easy—automatic even!—to locate a standing-still creature visually if the standing-still creature's not hiding or doesn't possess total cover or total concealment. (Also see this question.)
…But it doesn't cover every situation…
What the Dungeon Master's Guide on Invisibility fails to address is what happens if the foe does not or even cannot take that free action to make a Listen skill check (e.g. the creature possesses the condition nauseated (PH 310)). In that case, I refer the reader to Anaruoch: The Empire of Shade that, for context, early on includes a read-aloud description that's excerpted below:
[O]n the walls [of this room] hang wooden plaques supporting the dusty heads of various long-dead creatures—stags, boars, wolves, and other trophies. (9)
Then the DM is provided with the following rules:
Four of the creatures mounted in this gallery are still alive. [The antagonist's] four werewolf companions have thrust their heads through holes cut through the wall and the plaques, and they are posing as trophies until she gives the signal for them to attack. [N.b. What could possibly go wrong?] A successful DC 20 Listen check detects the sound of their breathing. Only if the PCs create adequate light to see with can they determine that the wolf heads are less dusty than the rest. (ibid.)
Keep in mind that Anaruoch offers no rules for changing this Listen skill check DC for distance despite the room's dimensions of 40 ft. by 30 ft., and there are no rules for how often the PCs get to make this Listen skill check! That makes these rules into guidelines, but they may serve as a starting point for the DM that needs to codify such rules for the campaign.
…Like retrieving an item from a backpack
With all this out there—and because there's no bonus on Move Silently skill checks for standing still—, retrieving an item from a backpack should probably be louder than just standing still. This DM would have a creature that takes a move action to retrieve silently an item from a backpack make a Move Silently skill check with the typical −2 penalty as per the DM's Best Friend (DMG 30). Further, much like moving at half speed incurs no penalty on Move Silently skill checks, this DM would allow a creature to take a full-round action (i.e. essentially two move actions or twice as long) to retrieve silently an item from its backpack yet suffer no penalty on its Move Silently skill check.