Spell attacks while hidden

Question

I've had a thought experiment regarding spellcasting, targeting, and hiding. It's known that casting a non-attack spell while unseen is not perceptible by the target, regardless of whether the spell succeeded or not (i.e. scry). However, what about spells specifically with the "spell attack" descriptor?

Here are the relevant rules.

Unseen Attackers and Targets:

When a creature can’t see you, you have advantage on attack rolls against it. If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

Casting a Spell, "Targets":

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.

Question 1: Does a hidden spellcaster that uses a non-obvious spell attack remain hidden? Let's assume the target does not notice the casting, either via subtle spell or otherwise (maybe they're in a loud and crowded tavern).

Question 2: If there are saving throws related to question 1, does the target know if it failed or succeeded?

Example A, Ray of Enfeeblement:

A black beam of enervating energy springs from your finger toward a creature within range. Make a ranged spell attack against the target. On a hit, the target deals only half damage with weapon attacks that use Strength until the spell ends.

At the end of each of the target’s turns, it can make a Constitution saving throw against the spell. On a success, the spell ends.

Scenario: A hidden sorcerer uses subtle spell to cast ray of enfeeblement at a target. For the purposes of this example, combat has not started yet (so the target is not hostile) and the target does not see the black ray. Does the target know it was enfeebled? If the target remains ignorant, does the target know it made a saving throw after 6 seconds?

Example B, Plane Shift:

You can use this spell to banish an unwilling creature to another plane. Choose a creature within your reach and make a melee spell attack against it. On a hit, the creature must make a Charisma saving throw. If the creature fails this save, it is transported to a random location on the plane of existence you specify. A creature so transported must find its own way back to your current plane of existence.

Scenario: A hidden wizard uses a familiar to deliver the touch spell on a target. If it succeeded its save, does it know it was targeted?

Answer 1

Attacking while hidden reveals you

1.) As you quoted, the PHB clarifies what will cause you to no longer be hidden. On page 195, it states (bold added):

If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

This rule does not distinguish between various types of attacks (e.g. melee, ranged, unarmed, weapon, spell), so any attack at all made by a hidden creature will give away its location and cause it to no longer be hidden.

Similarly, the text on the Feat "Skulker" in the PHB provides the following (PHB, p. 170, bold added):

You are expert at slinking through shadows. You gain the following benefits:... When you are hidden from a creature and miss it with a ranged weapon attack, making the attack doesn't reveal your position.

This is an example of an "exception that proves the rule." By pointing out that attacks that miss by attackers who have the Skulker Feat do not "reveal your position," this Feat lets us know that "reveal your position" and "give away your location" have the same meaning in the context of the rules.

In this context, this means that making an attack roll while hidden, including a spell attack, will reveal you.

2.) You also asked if a spell has a saving throw, whether or not the target of the spell will know if they succeeded or failed at it. There is no relevant rule declaring that hidden or unhidden spellcasters reveal the results of saving throws when they cast their spell, so in the absence of such a rule the answer is no (unless the failed saving throw has an obvious result, such as banishing the target).

Answer 2

The attack rules state that attack rolls as part of spells are attacks too: "Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure."

If your use a spell that requires an attack roll while hidden, you give away your location: "If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses."

If your spell requires someone to make a saving throw, they are going to know that something is happening. The rules state that saving throws "represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat." While you can argue that this resistance may sometimes be passive and without the character's knowledge (do you know if you are currently resisting the flu?), pragmatically if you call on someone to make a saving throw, they are going to realize there is something going on.

When it comes to spell effects, these are things that are physically affecting the characters. It's a bit unreasonable for them not to notice debuffs, they can feel it.

Remember that in 5e you normally won't tell the character what spell is being cast, you describe what the characters are experiencing. So they may be able to guess from the effects, but they won't know for sure.