OK.. We have an enemy creeping up on the party. Lets call him "Bwaha the invisible Assassin". The parties best passive Perception is 15. Bwaha's Stealth is 15. At what range does the character with Perception 15 detect the presence of a stealthy invisible creature tip-toeing towards him ?
Lets also state that it is mid-day, in a large open area with no obstructions. At what range can you "sense" the presence of Bwha the Invisible ?
I'd give the party disadvantage on their Perception checks to notice, which applies a -5 penalty to their passive Perception score. So they don't detect the Assassin at all.
That's cool. But at what range can PC's start making their Perception checks.. When the enemy is 30 feet away, 50, 100, 500?.. If a PC does detect the invisble creature, at what range would he be able to first notice him?
An eagle barbarian can see up to a mile away with no penalty to Perception checks.. So 1 mile for him? 
As the Stealth and Passive Perceptions are equal, the status quo remains. Bwaha is being stealthy and the PCs are remaining alert.
If the intent is for Bwaha to engage the PCs in combat I would call for a Perception check. Those that fail will be surprised. Those that pass aren't, but that doesn't mean they know Bwaha's exact position, just that something's put them on alert (though they can use their turn to try and figure it out if Bwaha hasn't struck yet). You don't worry about position until after initiative is rolled.
If the intent is something else, then that'll be played out as it unfolds. Either continuing to use Passive Perception or calling for an ability check to be made. The PCs won't notice Bwaha until something else happens to change the status quo. I wouldn't worry about position until that something exciting happens.
brettlangbdl wrote:
Rather than going by fixed "ranges", I go by reaslism. If someone is 10ft next to you but behind a solid wall, you have no change and spotting him. Down a well lit hallway without any obstacles, you can probably see as far as the hallway goes or at least until it curves too much (linear line of sight requires).
As for spotting an invisible unit, light and obstacles don't really matter. Instead it matters when you could hear a noise. Which depends ALOT on the location. A cave with high walls where all sound echoes through the whole cave probably gives away that there is someone else closeby at quite a large distance, even though you will not really know where the sound came from due to the echo-ing. Behind a wall that doesn't let sound through well? Probably impossible to spot. Of course it also matters on what kind of floor it is. Does the floor create a lot of noise or not so much? etc.
The moment I start thinking the PCs have a chance at all to somehow hear or see the enemy, I roll enemy stealth the first time. The result tells me if they spot the enemy or not. If the enemy is not invisible they will see it on a failed stealth roll, if the enemy is invisible they will only hear it (and I'd probably apply -5 from disadvantage to passive perception in this case).
I would also agree that it's very dependent on the environment. Invisible creatures can't be seen directly but may reveal their presence by making a sound or leaving other signs of their passage.
For example,
- if you are outdoors even a light wind will hide a lot of sound.
- A light snow cover could make it very difficult to sneak up on someone.
- A large open area with stone floor may echo more.
- Does the sound effects guy add in the "schinnng" sound you alway hear in movies (they don't really make that sound in real life) when the assassin draws his sword?
- Does the assassin roll a 1 on his stealth check and need to stifle a sneeze or knock something over?
However I would still give the PCs some chance to notice the assassin coming. Even if conditions are near ideal I would still only give the assassin advantage and the PCs disadvantage (or -5). They may not know exactly what is going on, but they should have a chance to detect something odd, even if only at a subconscious level.
As a general rule of thumb as DM I would allow the check at around 20 - 30 feet depending on environment since that's roughly the distance I might hear unusual footsteps or creaking of leather in a quiet environment.