As I play it, the difference would be that the adventurers would either have a percentage chance to encounter a patrol, or I would plan the route of the patrol and compare that to the route taken by the players' characters to see if they connect in space-and-time.
In the first instance, it becomes a matter of chance rather than certainty - in the second, it puts the outcome firmly in players' hands by making it a consequence of their choices. Either way I'm not deciding, "The adventurers will encounter a patrol as they attempt to escape..."Speaking for myself, that breaks down my suspension of disbelief - I suppose one could argue that the guard patrols are made up only of warriors who've demonstrated acute senses or whatever, but it strikes me as highly contrived. If I'm the GM in this case, I might give one or two patrol leaders some better than average skills or some other advantage, like trained wolves for tracking or somthing magical like an invisible stalker, rather than creating a corps of guards selected for the skills that I need to hose the adventurers. It's a slight distinction, I know, but it's one that is part of the verisimilitude of the game-world for me.
To me, this is not about 'illusion', but about providing real elements of chance and the ability of players to make meaningful choices that affect the outcome of encounters, rather than plugging in encounters because they 'make a better story'. I know it's a very old school approach, but I'm a very old school GM, so that just sort of follows I suppose.