Maybe I am reading some of these posts incorrectly, but there seems to be some consensus here that passive perception is something you have to "actively engage in" . Doesn't that pretty much ignore the word passive ?
Passive ...Definition = not participating readily or actively; not involving active participation - This is the dictionary meaning of the word passive.. Seems pretty clear you aren't required to take any action, or to state any intention you are ingaged in Perception, but that it is simply what you are aware of at any given time so long as you are conscious and aware ?
To require a character to state that they are engaged in Perception of their environment - That is an actual Action and is called the Search Action under available Actions, and allows a creature to make an active Perception skill check in combat as there action.
If a creature was required to say they are alert/engaged in Perception activity, rogues/ambushes would surprise you 99% of the time.. Makes rogue types pretty damn awesome. Also, would mean the first feat I took in your campaign would be Alert, which is an awesome feat anyway, but in that style of campaign, be pretty much mandatory!
Mike Mearls answered some questions about passive vs active perception on a Reddit AMA https://www.reddit.com/r/rpg/comments/2l69tp/ama_mike_mearls_codesigner_of_dd_5_head_of_dd_rd/
Someone asked:
A couple questions about passive skills. As I understand it, they are used when someone is passively doing something rather than actively, so passive perception is used as a baseline for how much a character notices when he's not actively looking around.
So if a character with a passive perception of 13 is actively looking around for something that can be seen on a perception DC of 10, and he rolls a 2, does he not see it? Or should passive perception be considered the minimum possible value for that character?
He replied:
Any skill can be used passively - it's up the DM to apply that as needed.
For perception checks, you passive result is always in effect. If you could see something with a DC 10 check and your passive is 11, you see it without rolling.
Keep in mind, though, that a DM might rule otherwise. Passive checks are a tool that groups can use to speed up the game or move past die results that slow things down or lead to a grind.