Why would the encounters that the PCs have be independent of the choices made by their players? I said I don't like random encounters or "wandering monsters". You are the one who has introduced the notion of "planned encounters" - all I've said about planning is in post [URL=http://www.enworld.org/forum/usertag.php?do=list&action=hash&hash=86]#86 [/URL] upthread, where I said
Overall, I keep my planning reasonably fluid, because I prefer to respond to what the PCs do, and what happens as a result - and find that this provides enough "randomness" or "unexpectedness" in outcomes.
Maybe 0%, if the discovery would add nothing to the game. Maybe 100%, if it obviously would. Maybe something in between that, if the "hanging around" is being resolved as a skill challenge, with being discovered one possible consequence of failing a skill check (the example I have in mind is Frodo and Sam "hanging around" in Mordor after inadverently engineering the deaths of all those orcs in Cirith Ungol).
I said a bit about how I would handle this sort of thing in post [URL=http://www.enworld.org/forum/usertag.php?do=list&action=hash&hash=86]#86 [/URL] also.
EDIT: Ninja-ed by [MENTION=16586]Campbell[/MENTION], although I think I might plan a little bit more than that. I often have ideas for thematically appropriate adversaries that also fit into the current general direction of ingame events, and will have them statted up, but will make decisions about where or exactly how to introduce them based on the dynamics of play. But Campbell is right that 4e's statblocks make it easy to introduce new or different adversaries easily. And it is also easy to level creatures up - if I statted up the hobgoblin captain 3 months ago, expecting that the PCs would confront him then, and then for various reasons that confrontation has been delayed for multiple sessions, I might still want the captain to be a worthy opponent even though the PCs have gained a level or two. 4e makes it very easy to do this.
We play the game in a totally different way. I run an organic world the PCs are not the center of the universe in my games. They maybe the ones on center stage but the world does not revolve around them.
If they going through dangerous enemy filled territory and they kill a bunch of orcs and then dally in the area there is a good chance that they will be discovered. If they kill the guards at the entrance of the keep and then go rest with plans to come back later they will find they have kicked a hornets nest over.
This is not railroading or taken players choices away from them this is having the world react in an organic fashion towards the PCs actions.