You can't 'decide there is no secret door in the room, based on my prep.' Not if you are playing Dungeon World according to the rules, which tell you not to decide the story ahead of time. You're allowed to have 'prep', but I put it in quotes because it is a very different beast from Gygaxian prep. It is a storehouse of potential lore that you can draw from in play. This is not meant to define what is possible, just to be resources to use. If, for whatever reason, the principles, practices, and agenda says "there really aught to be a secret door here" then it would be a misplay to not depict it.
Honestly, I've never seen a situation where an element as specific as an individual secret door was really mandated. You will always have choices. So it becomes a bit of a spherical cow at this point. But the general idea holds, prep isn't a set of constraints on what fiction can be presented, or how the story will go. Its more like a restaurant menu that you can pick from, which is a lot more convenient than just being prepared to cook any possible thing.
Ideally the GM's prep is also helpful in terms of allowing the GM to be a bit more pro-active and, for example, be pretty sure that some orcs are going to show up later. Maybe we don't know exactly when or where, and what will happen, but they're in your prep, so you probably WILL use them. Now you can leave some hints, or maybe you picked prepping orcs because Graaaaaahhhhh! Foecrusher expressed a hatred for orcs and it will be fun to see that in action.