In 5E, invisible creatures are not hidden by default. Even though you're invisible, until you take the hide action, your location is obvious due to your sounds, tracks, etc... We don't all like the rule, but it is there.
What about invisible objects or spell effects? If a cup is made invisible, what needs to be done to detect the presence of it on a table? What does a creature need to do to detect an arcane eye or rope trick portal?
From what I can tell, you fall back to generic rules of setting a perception DC to perceive something that is hard to spot under the RAW - perhaps a DC of somewhere between 15 and 25. Is that how you'd handle the PCs attempting to detect an arcane eye, a rope trick portal, a scrying sensor or an invisibile object on a table?
Consider a cup. If it's not moving and is invisible then sight won't reveal it's existence.
Exactly. Unless you bump the table and hear something fall over or see a clean circle on an otherwise dusty table or cobwebs attached to nothing you aren't going to detect it.
Unless, of course it starts talking to you.
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My problem with invisible items is that once players learn of their existence there are simple mundane ways to check for them at the cost of a trivial amount of time.
That and as was posted above, what purpose does the item serve in the game? Even assuming it's just a "nice to have" the DM sets whatever DC makes sense and then justifies being able to detect it because of some external environmental factor. That can be anything from automatic (you touch it after you see a noisy bird sitting on nothing) to nearly impossible (moats of dust in the air moving oddly).
In 5E, invisible creatures are not hidden by default. Even though you're invisible, until you take the hide action, your location is obvious due to your sounds, tracks, etc... We don't all like the rule, but it is there.
What about invisible objects or spell effects? If a cup is made invisible, what needs to be done to detect the presence of it on a table? What does a creature need to do to detect an arcane eye or rope trick portal?
From what I can tell, you fall back to generic rules of setting a perception DC to perceive something that is hard to spot under the RAW - perhaps a DC of somewhere between 15 and 25. Is that how you'd handle the PCs attempting to detect an arcane eye, a rope trick portal, a scrying sensor or an invisibile object on a table?
In the case of an object created or made invisible by a spell, wouldn't the caster's spell DC be the obvious choice of DC to detect its presence, provided there is a plausible source of evidence about its existence?
Perception checks based on sight automatically fail, but if the object is moving (like an arcane eye), there might be disturbances to the air, which perception (at disadvantage) could pick up on. But then figuring out what it actually is might require further action (detect magic, see invisibility, maybe an arcana check to make the inference).
If it's a static object in a room, I'd probably reveal its existence as part of a sufficiently high investigation roll to search the room rather than a perception check, since finding it is more of an inference than it is a sensory thing.
But in each case, I'd use the caster's DC, with advantage or disadvantage applied to the check depending on the situation.
IMO. It's either invisible or not. Better casters don't make objects more invisible than worse ones.
I wouldn't unless they had indicated they were investigating the table the invisible cup was sitting on and doing more than looking at it while doing so.