I have been looking at the Ranger Horizon Walker subclass and I was wondering if Detect Portal can detect a Bag of Holding and other such items?
From what I understand a portal is a link between two stationary points on a plane. Since you can always access the stuff in your bag, I can assume that the bag links to a stationary point and the bag’s entrance is stationary in relation to the bag, but the bag as a whole moves.
Does Detect Portal work to find or detect Bags of Holding?
The Horizon Walker Ranger's Detect Portal feature states:
At 3rd level, you gain the ability to magically sense the presence of a planar portal. As an action, you detect the distance and direction to the closest planar portal within 1 mile of you...
See the "Planar Travel" section in chapter 2 of the Dungeon Master's Guide for examples of planar portals.
The section on "Planar Travel", specifically the section on "Planar Portals" states:
"Portal" is a general term for a stationary interplanar connection that links a specific location on one plane to a specific location on another. Some portals function like doorways, appearing as a clear window or a fog-shrouded passage, and interplanar travel is as simple as stepping through the doorway. Other portals are locations-circles of standing stones, soaring towers, sailing ships, or even whole towns-that exist in multiple planes at once or flicker from one plane to another. Some are vortices, joining an Elemental Plane with a very similar location on the Material Plane, such as the heart of a volcano (leading to the Plane of Fire) or the depths of the ocean (to the Plane of Water).
The requirements for a Planar Portal:
1. It must be a "stationary interplanar connection"
2. It must link a location on one plane to a location on another plane.
A "stationary interplanar connection" would be like a stationary connection between two cellphones. It does not mean that the phones are not moving, it means that the connection is stable, this is supported by the fact that locations which change planes or exist in multiple planes count as portals, especially when it can be a moving, sailing ship.
We know that a Bag of Holding is an extra-dimensional space as shown in the question "Is the interior of a Bag of Holding actually an extradimensional space?". The Portable Hole item states:
[...] Placing a portable hole inside an extradimensional space created by a bag of holding, Heward's handy haversack, or similar item...
Additionally, extra-dimensional spaces and demiplanes are the same things in fifth edition as shown in the question: "Is the “pocket dimension” a familiar goes into a demiplane or an extradimensional space?".
Thus the inside of a Bag of Holding is a demiplane; it is its own plane of existence. We also know that it is a constant/specific plane of existence as the contents of the bag are always all there. The problem though is this: Does the bag itself count as "a specific location on one plane"?
What ultimately counts as "a specific location on a plane" will be up to your GM; however, we do have some examples that are mobile: sailing ships or towns, if they exist in multiple planes, or change planes.
A Bag of Holding does not do anything more drastic than these locations, and the connection is no less "stationary".
That said, a ship and a city are certainly locations, but calling a bag (though a magical one) a location isn't a particularly natural feeling statement.
If a Bag of Holding were considered to be a location, then it would qualify as a Planar Portal and thus be detected by Detect Portal.
Note that Detect Portal only detects a portal within 1 mile. What 1 mile means when crossing planar boundaries is not defined by the rules, so how Detect Portal works if a portal is on a different plane of existence would be up to a GM.
Also as user @Black Spike points out, Detect Portal only finds "the closest planar portal within 1 mile of you", and so it will always find only the single closest. If the party were currently carrying around something like a Bag of Holding then the Ranger would have to go far away in order to effectively search for portals.
The following assumes Detect Portal will find Bags of Holding and addresses when it will actually do so:
Thanks to user @Andrew Biggins for finding this question "Does Portal Lore detect currently inactive portals?", the answer there explains that Detect Portal makes no requirement that the portals themselves be currently active. Thus it will work on portals even when they are no longer active.
One odd scenario is when the bag is placed into another extra-dimensional space:
Placing a bag of holding inside an extradimensional space created by a Handy Haversack, Portable Hole, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it to a random location on the Astral Plane. The gate then closes. The gate is one-way only and can't be reopened.
The gate still connects both locations, it is simply a one-way connection, and so one end of the gate would be detected by Detect Portal, but the other would not.
The Horizon Walker ranger's Detect Portal feature detects portals as they are defined in the DMG (See the "Planar Travel" section in chapter 2 of the Dungeon Master's Guide for examples of planar portals.). The definition in the DMG for a portal is:
"Portal" is a general term for a stationary interplanar connection that links a specific location on one plane to a specific location on another.
For terms that are not defined in the game, we use the dictionary definition. The dictionary definition for "stationary" is:
not moving or not intended to be moved
A bag of holding — including the opening — however is intended to be moved, and often is moving, so it is not stationary. The very point of it is that you can carry it around. Since only stationary interplanar connections count as portals, a bag of holding is not a portal. The same would apply to similar items like a portable hole, or a handy haversack.
The only ambiguity here comes from another type of portal in the DMG including sailing ships in its list of examples:
Other portals are locations — circles of standing stones, soaring towers, sailing ships, or even whole towns — that exist in multiple planes at once or flicker from one plane to another.
Such portal locations are described to exist in multiple planes at the same time, or flicker from one to the other. A bag of holding does neither. The interior space of the bag is described as an extradimensional space, so it never overlaps with the current plane of existence or intermittently enters it, and this definition of portal does not apply to it.
There are also many questions about sailing ships moving in relation to requirements about movement in spells (also here, and here), and the consensus is that while up to adjudication by the DM, generally things like a sailing ship that are large enough to be used as a frame of reference in a scene probably should be considered stationary when it comes to interpretation of such rules.
