Not every Encounter needs to be against opponents who have seen you yet. Your escape from zombies in particular may be made difficult only because they inadvertently wandered into your location. In this case, it is entirely possible to escape such an Encounter without alerting the undead (or any other opponent) to your presence. This is, obviously, easier said than done.
What this is essentially about is roleplaying the efforts a character (or party of characters) is making when they make a ‘Per - Hide’ check to avoid an Encounter. It is basically the same thing to have characters describe in detail how they are sneaking about, but it is far more exciting than just making a single check and extrapolating the results from there.
Players may decide to enter an Encounter against un-alerted opponents in an effort to sneak past them or to take them out quietly. Either way, it’s a tactical option and is treated as such, complete with increased chance of gaining Risk as well as earning SP rewards.