In the D&D context, the best-developed way of handling this is via skill challenge. When the players at
@Hussar's table fail their third check (while doing reconnaissance or whatever) the GM narrates the consequence. That might be an immediate attack; or the GM could say - "OK, you've set up your positions, but after an hour or two of waiting no one's turned up yet - and then suddenly you hear the sound of soldiers approaching from behind you!" It is the
mechanical failure that gives the GM licence to narrate without railroading.