Palpitine Hid In Plain Sight... But We Still Not Sure How
Though it's easy to hurl fingers at the Jedi for not seeing sooner that Palpatine was evil, he most definitely managed to hide himself in the Force. Master Yoda's power was great enough in Star Wars: Episode II - Attack of the Clones to perceive Anakin Skywalker's suffering on Tatooine throughout the Tusken Raiders event all the way from Coruscant. Yoda wouldn't have missed recognized Palpatine's malevolent presence in the Force given it is almost impossible.
Presumably, Palpatine employed one of the Force powers in Star Wars, most likely Force camouflage. This capability, as the name suggests, allows one to hide themselves in Force, therefore producing a kind of void. This would most definitely help to explain why, despite their regular cooperation, the Jedi never found Palpatine as a Sith Lord. Even for a strong Sith Lord like Palpatine, this would be quite a challenge, though, and Star Wars has not yet verified exactly the tactic he followed. Still, this theory seems to be the most feasible one thus far.
The Stranger Revealed Cortosis Can Be Used To Hide In The Force.
The Acolyte made it rather plain how the newest Sith, Qimir/the Stranger, managed to go undetectable, even though Star Wars not yet reveals how Palpatine pulled this happen. Qimir specifically chose cortosis, a metal from Legends that could short out lightsabers and, clearly, mask one's existence in the Force. As only Qimir's old Jedi teacher, Vernestra Rwoh appeared on Brendok and promptly pulled his helmet on, hiding himself once more, the last power was especially on show in The Acolyte.
Still, the potency of Qimir's helmet probably went much beyond what the show immediately observed. After all, although Vernestra sensed Qimir on Brendok when she arrived and his helmet was off, plainly, until Jedi were being slaughtered, no one sensed the mounting evil coming from Qimir (or, importantly, Darth Plagueis) in the Force. Presumably, Qimir had been wearing this helmet for some time, able to go unseen by the Jedi even while he developed his evil side Force might.
This would also help to explain why, despite their obvious presence in the shadows, the Jedi thought the Sith were extinct. Since many thought it to be a retcon, this was a main point of conflict around the program. Ki-Adi-Mundi had said in Star Wars: Episode I - The Phantom Menace that the Sith had been extinct for a millennium, implying the Jedi had no knowledge of the developing threat whatsoever. Many viewers were enraged and debating that it was a retcon with Qimir's entrance on the scene.
Still, cortosis would most certainly help to explain this. Actually, according to showrunner Leslye Headland of The Acolyte, the Unknown Planet—which stayed nameless even in the show's finale—was supposed to possess cortosis. This implies that the Sith would find the ideal hiding place here, given the ability of camouflage via cortosis.
Why didn't Palpatine employ cortosis?
Given the obvious potency of this metal and Palpatine's need to blend in plain sight, it's strange he neglected cortosis. Still, the solution perhaps lay in the subtleties of Palpatine's proposal itself. Although cortosis can obviously hide someone in the Force, Qimir's hurried attempt to put the helmet back on his head upon Vernestra's arrival implies that the helmet must be worn if concealment is to be successful.
Although Palpatine would have found such a helmet useful in some circumstances, his greater purpose would have been undermined by the necessity of wearing a helmet to conceal himself in the Force. Palpatine wanted not to rule with brutal might. Instead, he developed a very particular scheme whereby he would enter the Senate and the Jedi Order under the guise of a sincere politician serving the galaxy during conflict. Actually, this strategy was quite brilliant and shows how much forethought Palpatine truly possessed.
Had Palpatine had to assume a helmet, this accomplishment would have been undone. Palpatine would not have been allowed to be a senator at all in fact with a helmet like Qimir's. His charisma helped him to have a more effective plot, so his bids for his several positions in the Republic Senate were partly justified. Actually, after his visage in the original trilogy, Palpatine was absolutely unidentifiable in The Phantom Menace.
It was clear in The Phantom Menace why Palpatine was so much admired and trusted. Acting to help Queen Amidala, he seemed sincere and even kind in that film. Clearly, wearing a mask—especially a scary one—he would not have had the same effect or been seen in almost the same manner. Furthermore, for the Jedi, who might even have known about such a thing, such a mask would have much more quickly caused worry. Palpatine's need for proximity to the Jedi would have stopped his scheme from coming to pass.
Arguably, Palpatine's mask may have looked somewhat different from Qimir's, and maybe he could have made that work. Still, his plans revolved on his ability to hide in plain sight, expose his face, and seem like someone deserving of trust and authority regardless of how he built his mask. Ultimately, it seems logical that Palpatine chose not to wear a cortosis mask like the one in The Acolyte, even if it would have been a somewhat cool turn for his Star Wars narrative all the same.
