What you're saying makes sense for sure. But doesnt the same logic mean that he should have been able to sense the Grandmaster of the Jedi, Yoda? It's most powerful warrior in Kenobi? You'd think he would have scanned the Force for those two specifically, no?
Palpatines great evil was playing both sides and creating a galactic-wide civil-war purely so he could end up as the Emperor. It's not far fetched that he could have been as blinded by his need for power as Plageuis was by his need for immortality.
Also, perhaps Plageuis was able to become one with the force ala the Jedi, and that WAS his immortality. And therefore, Palpatine may have not been able to sense him.
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