Dnd-5e – Are Beholder’s Rays Visible and Audible? Explanation Based on Rules, Lore, and Novels

Combat is frustrating when you are unable to act. Combat is challenging when you are threatened, at risk, but able to respond.

In order to reduce frustration, make things that reduce actions with save-ends effects staged. A stun or dominate should never be the first thing dropped on someone with a save-ends (End of next turn is fine, however.)

I prefer using the progression of: Granting Combat advantage, dazed, stunned for my save-ends effects that were originally stunned, and dazed -> Dominated for my dominates, especially if they originate from an at-will.

The best way to make combat challenging and exciting is to scatter terrain features that both allies and enemies can use to great (and lethal) advantage while stationing fairly beefy enemies at them. Then the combat has a sub-goals of, for example "control the [really nasty thing shooting fire at us] and reorient it so we can rain fire on the enemies" as opposed to the "clear the battlefield." By increasing tactical complexity, you increase the challenge, but also increase the opportunities for innovative tactics and player self-expression.

Xanathar's Guide to Everything has optional rules for this situation

Dungeon Master's Tools > Spellcasting > Perceiving a Caster at Work

If the need for a spell’s components has been removed by a special ability, such as the sorcerer’s Subtle Spell feature or the Innate Spellcasting trait possessed by many creatures, the casting of the spell is imperceptible. If an imperceptible casting produces a perceptible effect, it’s normally impossible to determine who cast the spell in the absence of other evidence.

The actual casting of the spell is hidden¹ by virtue of the subtle spell metamagic.

The effect of casting the fireball spell is to create

A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame.

The streak of light coming from your finger is pretty clearly "other evidence", however it has some caveats as to how strong this evidence is. Your finger does not have to be pointing from an outstretched arm, not does it need to be obvious.

The bright streak however is an obvious and perceptible effect of the spell. If the Sorcerer was behind the cleric the cleric would absolutely notice a streak of light coming from behind them, but they may not notice who it came from. So would the majority of the rest of the party.

_{1. Note there is a potential argument to be made that the arcane focus is still perceptible under normal circumstances, however the special ring item you have created for the PC removes this from consideration from a detection perspective.}

Passive Perception does not give you 360 degree vision

PHB > Ability Checks:

Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.

If the Sorcerer were in any way intelligent they would know the streak of light is produced by the fireball and would wait to cast it until no one in the party was looking at them.

Thus the cleric would not "automatically" know where it is coming from due to their passive perception.

Casting fireball is not an attack

The rules you mention about unseen attackers state

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

The game rules define attacks pretty clearly (in the same section of the PHB as the unseen attackers rules).

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.

Fireball requires no attack roll, thus it is not an attack.

As a result the unseen attackers rule section does not apply.

Would a subtle psychic scream be imperceptible?

Yes it would. It requires only somatic components and produces no perceptible effect that would indicate it's point of origin.