iserith
Magic Wordsmith
I just keep that simple. Moving at half pace gives passive checks. Actively searching for secret doors (ie the party stops and does their thing) I would agree would cause the PC doing so to lose that perception for a kobold sneaking up.
The rogue scouting ahead and disarming a trap or opening a lock would be another example that I think I see more often. That rogue could have an awesome passive perception but if he's focused on disarming a trap then there's an argument against using it while those kobolds sneak up behind him. Blindsense starting at 14th level is more applicable in that situation.
Okay. The way I look at it is kind of a "zoom in" or "zoom out" situation. So if you zoom in, you might have characters declaring specific, say, "exploration tasks" in a given area. Being that those are not repeated tasks, if there's any uncertainty and a meaningful consequence for failure, we'll have an ability check. If we're zoomed out, characters are covering a lot of ground, performing tasks repeatedly in the doing, and we're looking at things from a high level as with Activites While Traveling. That's when things get resolved with passive checks.