In our 4E D&D game, we handle this through a Skill check by a PC during a Skill Challenge, whereby the player determines the fiction on a successful roll and the GM determines the fiction on a failed roll (as a complication to be met rather than negation of player's idea). Here's an example from our PbP:
GM:
Its the season for imports from the network of enclaves all around The Empire. Imports means audits and taxes at the gates. The priests, the blessed, and the scrivener arrive to a mass of exhausted merchants, farmers, artisans, fresh from disembarking their caravan with their beasts of burden and wagons being serviced and cared for by stable hand, farrier, engineer. Auditors and tax collectors, like vultures to a carcass, descend upon them. Imperial Sentries mill through the throng, ever watchful, imposing stares, armor, weaponry creating an intentional mood of fear, acquiescence, and order upon the proceedings. The two stone towers loom with more eyes, the ramparts between them yet more eyes, bows, arrows.
The drawbridge that spans the moat is up. The gatehouse door is locked with an Imperial Guard manning his post there, keys dangling from his belt.
MEDIUM DC
How do you get beyond the soldiers, the wall and its moat, and into the wild?
Player:
The priests, the blessed, and the scrivener stand, dispirited, before the milling throng inside the Manticore Gate. On the one hand, the Imperial Sentries have their hands full maintaining order over the crowd, keeping tempers from flaring into violence, and overseeing a steady rhythm of coins collected for the coffers of the Empire. So there is little chance their small party will draw too much undue notice. Yet on the other hand, the gatehouse door stands barred; the bridge that would span the moat to allow passage across is raised; and no small amount of time will pass in processing this current multitude before either of the previous obstacles is alleviated in the admission of more importers and artisans from outside the city gate.
"This is not typical," says Xin Mae. "Is this typical? When I traveled here across the Sea of Silt and Sand from Kifu Chanyee, I passed but minutes at the gate. I don't like holdups of any kind. Isn't there something you can do? Surely, there's something you can do," he pleads generally to the priests.
Ibhea explains patiently about the differences between Manticore Gate at import season and the Great Dragon Gate admitting infrequent voyagers from the south, but his discourse does little to assuage the scrivener's frustration.
Meanwhile, Arunny, white basleq drawn over the ivory wimple that shields her golden mane, scans the Imperial Sentries for signs of recognition among them of the now-fugitives. So far, no visible leaflets bearing their images seem to circulate, but for how long?
One Sentry draws her particular attention, his unshaven visage unfamiliar to her ... and yet, there is a ... something ... call it a shadow? a kind of vague outline ... present when he speaks that reminds her of Ibhea somehow. She indicates the Sentry to her fellow with a subtle gesture, "Do you know that man?"
IBHEA's brow creases as he regards the Sentry, and then his eyebrows lift in recognition. "Why, yes, Arunny! In fact, I do!" He throws the hood of his scarlet cape over his golden tunic and pushes his way through the crowd, receiving not a few looks of ill regard at his presumption, despite the robes of office.
In short order, Arunny watches as the Imperial Sentry drops his stoic mask and grasps Ibhea in a full embrace, pounding his back in familiarity. The two men have a brief conversation, and Ibhea points back at their party twice during it. When he returns, Ibhea explains that the Sentry, Kosal, is an old friend from childhood, one who escaped a life of poverty and desperation by enlisting in the The Guard in much the same way that Ibhea had by finding the faith. Kosal can get "the missionary team" through the gate, across the moat, and out of the city when the Manticore Gate is lowered for the next wave of entrants.
Ibhea makes a Primary Skill Streetwise check to find a familiar and useful contact within the assembly, thus identifying Kosal, +14 vs 15 Moderate DC = autosuccess to complete the Skill Challenge.