He was likely trying to sense where the Jedi enclave he'd been told about was. And confused as to why he couldn't sense it. Also he might not have been in range yet.
A single Jedi is beneath his notice. He was told that there were enough to satisfy his hunger.
So what? His ship may have still been too far away from the planet for him. It's not as if "around the planet" means he's right next to it, he's still hundreds of miles separate from it.
Nah, he doesn't notice Force-sensitives unless they are in large populations:
"He sees planets, stars, not people. To him, the planet below, the station with its teeming life, only that is massive enough to demand his attention."
―Tobin (Star Wars Knights of the Old Republic: The Sith Lords)
He didn't even notice the presence of Visas, Meetra, and Canderous when they came:
"I do not believe he knows his ship has been boarded, and if he did, he would not care. The extent of his power cannot be put into words, and his perceptions have grown as well. To him…"
"You are dust motes in a storm, a grain upon the beach, and as insignificant as a body that orbits the graveyard of Malachor."
―Visas Marr & Tobin (Star Wars Knights of the Old Republic: The Sith Lords)
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."
Indeed, and he was going to, just he was cut short by the arrival and confrontation with the Exile.
I suspect how long it takes him to drain depends on how much Force energy is in the planet. Katarr had much more then Telos.
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."
Not really. And it's not like he had to do it right away, no living being in the galaxy was even worthy of his recognition.
Trying to understand the actions of a being like him doesn't really work... he perceives time and the galaxy differently from others.
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."
"He cares nothing for the Sith or its teachings, or the Jedi. And when the Jedi are dead, he will feed on the galaxy, the Republic, and eventually, consume the Sith as well."
―Darth Traya (Star Wars Knights of the Old Republic: The Sith Lords)
"As Nihilus greedy consumes entire planets life energies, the dark side macerates him even faster."
―Knights of the Old Republic Campaign Guide
"Amazingly dark and devastating powers are the purview of some of the greatest Sith Lords of the Knights of the Old Republic era. They devastate and consume entire worlds with a thought. [...] Average beings stand no chance of stopping these ultra-powerful monsters of the dark side."
―Power Beyond Belief: Using Ultra-Powerful Sith Lords in Saga Edition
"Darth Nihilus began his Jedi purge by obliterating the planet of Katarr, where a secret conclave of the most powerful Jedi were taking place."
―The Complete Star Wars Encyclopedia
"The last known Convocation occured on Katarr some 3,952 years before the Battle of Yavin. The assembled Jedi were slaughtered when the Sith Lord Darth Nihilus laid waist to the planet."
―The Complete Star Wars Encyclopedia
"And it devours him as he devours others—his mere presence kills all around him, slowly, feeding him. He is already dead, it is simply a question of how many he kills before he falls."
―Darth Traya (Star Wars Knights of the Old Republic: The Sith Lords)
"Then her lies will mean the planet's destruction, he will destroy all of Telos. He will turn it to fire again and crush the planet beneath him."
―Visas Marr (Star Wars Knights of the Old Republic: The Sith Lords)
"If there are no Jedi here, then my Lord cannot feed his hunger. He will destroy the planet, the station, he will cleanse it of life. Even if the people below are not Force Sensitive, the small amount he can feed on from the mass destruction of the station, and the life of the planet, will sustain him a while longer."
―Tobin (Star Wars Knights of the Old Republic: The Sith Lords)
"Darth Nihilus is literally a destroyer of worlds, powered by an insatiable hunger that drives him to consume greater and greater populations. While one may reasonably expect someone fallen to the dark side to employ Force lightning against a foe, no one expects a fleet of starships commanded by Sith Lords consuming entire planets. Nor would Nihilus tolerate such a rival and competitor for his sustenance."
―Power Beyond Belief: Using Ultra-Powerful Sith Lords in Saga Edition
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."