While fighting through a wizard's tower, the party breaks into an unexplored storeroom to take cover from enemies down the corridor outside the room. Unbeknownst to them, there is an invisible skeleton standing against the far wall of the room with instructions to attack anyone who walks within 5 feet of its position. Due to the ongoing fight, the room is also subject to the effects of a silence spell, which will likely end in a round or two when the party shoots the mage maintaining the effect.

The problem is that the skeleton in the room has not taken the hide action, and thus is not hidden (this is because it is exploiting flaws in the command it was given to try and get its master killed), and thus I need to describe the situation in such a manner that the players are aware of what general space the creature occupies so that they can make their attacks with disadvantage against it if they want. It also seems like I need to give them some information about what it is because otherwise several of the party members will be unable to tell the invisible silent unhidden skeleton apart from the invisible silent intermittently unhidden party rogue, which seems contrary to the idea that people aren't allowed to be confused about what space you are in or what's in a space unless you take the hide action (see, e.g., Do you need to take the Hide action when you are Invisible, make no noise and leave no traces?).

As a result, I'm unsure how to provide appropriate description. The skeleton doesn't smell particularly of anything (and the party barbarian is currently coated in excrement from a wrestling match with an Otyugh in the basement), no one is touching it currently and it's not moving, there's no sound in the room and if there were the party nonetheless couldn't hear it, and furthermore they can't see the skeleton.

I've considered using slight temperature variations in the future to handle this (e.g. "you feel an almost imperceptible chill emanating from the middle space along the back wall as you enter this room"), but that relies on the creature altering the temperature around itself and while that works for ghosts it doesn't really fit with the way I've been running skeletons so far.

How can I best narrate the information I need to communicate here?

Visual effects are still probably going to be your best option

You have basically eliminated the possibility of using sound and smell from being easy (if not even possible) to use, and temperature seems like quite a longshot. So, your best options are likely going to be visual.

Just because they don't see the skeleton doesn't mean they aren't able to see that something might be there. Use some visual effect from the environment that interacts with the skeleton to show that something is there.

The wizard wants it to be hidden but the wizard isn't here right now and the command that was issued wasn't specific enough. The skeleton wants to fail.

Not taking the Hide action means that they make no attempt to hide their location from being known so there are many examples you can use to highlight the faults with that. Something that wants to be found would be even more careless about concealing its presence.

Examples

You say that the skeleton does not care about being hidden, so then maybe it would not care enough to clear the cobwebs that have formed in its ribcage (presumably after the skeleton's body has been turned invisible). Perhaps it cares so little about hiding that even a piece of clothing from the last victim is still clinging to its cold blade.

Maybe there is a suspicious lack of dust in the area around the skeleton if it stands in that one location the entire time. Or, if it moves, maybe there is a trail or groove in the floor from where it usually moves.

Perhaps there is a slight breeze is in the hallway (maybe stirred by the PC's motion) and it gets a banner hanging on the wall stuck on the skeleton. Or flapping in such a way that they notice something must be blocking it.

Things in the air might also help reveal it through their movement patterns (smoke or mist) or by condensing on the skeleton (fog, water). Though some of these would be pretty odd effects to have in a building room.

Your skeleton might have been hidden regardless

I'm not aware of all the minutia surrounding what exactly the skeleton would or wouldn't try to do or the limits of the command, but there is a case to be made that, outside of battle, simply being unseen and unheard is enough to be considered hidden without having to take the Hide Action explicitly.

Like most formalized Actions listed in the rules the Hide action is listed in the section called "Actions in Combat". In combat, detailing what you can do and how much time it takes is very important, but outside of battle is is not as much. And the rules aren't clear that tracking actions outside of battle is usually what is expected. Since the Hide action simply represents attempting to be unseen and unheard and conceal your location, doing so outside of battle probably doesn't require the explicit use of the Hide action. And, furthermore, it may be possible to be hidden as long as you are simply unseen and unheard.¹

Now, you imply that the skeleton may be disinclined to accept being hidden and that its instructions were not explicit in that respect. So, depending on the details, it is likely that you, the DM, would have to make the final determination on if what the skeleton does or does not do is enough to consider it not hidden. However, if the skeleton was perfectly still, unseen, unheard, unsmelled, etc and didn't do anything to change that there is a good chance that the rules would consider it simply to be hidden.

^{1 - thanks @MarkWells for outlining this idea in a comment under another answer:}

^{The Hide action is introduced in a section of rules titled "Actions in Combat". That's important. Actions are formalized so that we can prevent a creature in combat from doing too many things at once. But what the Hide action represents is getting out of sight and being quiet, just like the Attack action represents trying to strike an enemy with your weapon, or a Short Rest represents spending an hour doing nothing strenuous. If you get out of sight and be quiet and don't call it the Hide action, you're still hidden.}

Assuming that the Wizard who made the skeleton invisible did so using the Invisibility Spell (and let's further assume that they used permanency on it so that it would last longer than the 1 hour limit) or otherwise used a spell-like effect (such as a potion) that has the same language...

Anything the target is wearing or carrying is invisible as long as it is on the target’s person. The spell ends for a target that attacks or casts a spell.

The skeleton could make an attack against the wall to allow itself to become visible. Now it's completely undone the Wizard's intent.

Even using Greater Invisibility, which has a shorter duration of only 1 minute and would also need to be made permanent more than likely, the language regarding "Anything the target is wearing or carrying is invisible as long as it is on the target's person." is still there. As such, there are many ways the skeleton could make its presence known:

• throw items


• juggle its sword (or other weapons)


• move an item around


• etc...

Items would be visible while in the air, because they aren't on the skeleton's person during that time. This would be rather conspicuous...

I believe that arguments have also been made that only items on the target's person at the time the spell is cast are invisible. As such, the skeleton can simply pick up an item, and it would appear to float in the air. Skeleton puts on a shirt and some pants, and now it's clothing is visible, even if the bones underneath are not. At that point, I wouldn't even give the players disadvantage anymore...