The Xanathar Guide for everything has optional rules for this situation.
Dungeon Master Tools> Spellcasting> Perceiving a Launcher at Work
If the need for the components of a spell has been eliminated by a special ability, such as the Subtle Spell feature of the sorcerer or the innate spell feature that many creatures possess, the cast of the spell is imperceptible. If an imperceptible throw produces a perceptible effect, it is usually impossible to determine who casts the spell in the absence of other evidence.
The real cast of the spell is hidden.one Under the subtle metamagic spell.
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 blooms with a low roar in a burst of flame.
The light streak coming from your finger is clearly "other evidence", however, it has some warnings about how strong this evidence is. Your finger does not have to be pointing from an outstretched arm, it does not have to be obvious.
The bright streak though. It is an obvious and perceptible effect of the spell.. If the Sorcerer were behind the cleric, the cleric would notice absolutely a streak of light behind them, but they may not notice it. who He came from. That's what most of the rest of the game would do.
1. Note that it can be argued that the arcane focus is still noticeable under normal circumstances, however, the special ring element that you created for the PC eliminates this from a detection perspective.
Passive perception does not give you a 360 degree view.
PHB> Skill Controls:
Your Wisdom check (Perception) allows you to detect, hear or detect the presence of something. Measure your general awareness of your environment and the sharpness of your senses. For example, you can try to listen to a conversation through a closed door, eavesdrop under an open window or listen to monsters that move stealthily in the forest. Or you can try to detect things that are hidden or easily lost, whether they are orcs ambushed on a road, thugs hiding in the shadows of an alley or candlelight under a closed secret door.
If the Sorcerer were intelligent in some way, they would know that the streak of light is produced by the fireball and they would wait to throw it until no one at the party was looking at them.
Therefore, the cleric would not know "automatically" where he comes from because of his passive perception.
Throwing fireball is not an attack.
The rules you mention about invisible attackers indicate
If it is hidden, whether it does not see it or not, when it makes an attack, it gives away its location when the attack hits or fails.
The rules of the game define the attacks quite clearly (in the same section of the PHB as the rules of the invisible attackers).
If there is ever any doubt as to whether something you are doing counts as an attack, the rule is simple: if you are making an attack roll, you are making an attack.
Fireball does not require an attack roll, therefore it is not an attack.
As a result, the section of the rule of invisible attackers does not apply.
Would a subtle psychic scream be imperceptible?
Yes, I would. It requires only somatic components and does not produce any perceptible effect that indicates its point of origin.