D&D 5E Finding Invisible Objects and Spell Effects

[Oofta]

Has anyone had to rule on whether detect magic can locate an invisible item?

I’ve personally ruled that the individual can “feel” something hidden is in range, but its location cannot be pinpointed.

Detect Magic states

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.​

So yes, they would know there was magic in the area. Given enough time and depending on the location they may be able to narrow it down by walking around until the magic is just on the edge of the spell and triangulate.

 

[Maxperson]

Has anyone had to rule on whether detect magic can locate an invisible item?

I’ve personally ruled that the individual can “feel” something hidden is in range, but its location cannot be pinpointed.

Yes and I rule that no, it does not. The reason is that if detect magic worked on illusion magic, you really wouldn't have needed to develop see invisibility and the like.

 

[GMMichael]

As a DM, how do you determine if the PCs can perceive these things without the aid of see invisible or similar magics?

They can't be seen. That's what invisible means. But your second post says that the PCs are using these, so are you asking for NPCs?

Do you treat them like hidden objects and allow a perception check?

I'd want my spell-education money back if I paid to learn how to cast a detectable Arcane Eye.

Do you not allow it to be perceived unless someone walks into it (or uses magic to find it)?

I don't know. Maybe the rope trick portal is like the zipper of a tent; it's there, but you have to poke around to find it.

Or did I miss a rule somewhere?

Nope. I think the D&D designers want DMs to make a ruling on it.