Because Dumbledore was present, William ultimately did not succeed.
After a simple grouping of two people per team, William, Lupin, and Professor McGonagall took on the patrol duty of the fourth floor. After all, as Dumbledore said, Lupin's bandaged arm couldn't even hold a wand, so he and William were now considered a unit.
"That Sirius... can he really still be lingering in the castle?"
In the Charm classroom, William lifted the trash can at the back of the classroom and looked inside. It was empty — after putting the lid back on, he followed Lupin out of the classroom and into the hallway, where Professor McGonagall was checking a long row of armor —
Although they all swore there was no one hiding inside them, Professor Meow was evidently not reassured.
"They're all empty, Blake probably has already escaped from the castle..."
Professor McGonagall extended her Lumos Charm-lit wand into the gap of the last piece of armor, watching the light escape through the holes in the faceplate. She shook her head, "But that's just a guess. We must ensure the safety of the entire castle —"
Perhaps thinking William suggested this out of laziness, Professor McGonagall turned her head seriously.
"Hey, weren't you familiar with that Blake before? Where do you think he would hide?"
Continuing towards the remaining classrooms on the fourth floor, William nudged Lupin with his elbow, asking — McGonagall, who had been checking the ceiling along the way, turned to look at Lupin, knowing full well of Lupin's prior relationship with Sirius, but upon seeing Lupin's pale face, concern crossed the elderly professor's face.
Since Lupin entered the school, she had never mentioned the other three Marauders in front of him, because, at the time, as the Head of Gryffindor House, she understood the depth of feeling between the four, even though they caused her plenty of headaches during their school years...
Alas, things have changed...
As Professor Meow sighed at those gone, Lupin did think carefully for a moment before shaking his head, "No, if it's the Sirius I remember, he definitely wouldn't stay in the castle... but, I don't know if I can trust my memory."
Lupin's tone carried bitterness, clearly filled with sentiment...
He once mentioned to William what Sirius used to be like, "Honestly, I still can't figure out why he would betray us..." Lupin sighed, as the three of them finished scouting the fourth floor, standing before a statue of a one-eyed witch —
"...I remember this place."
Noticing the atmosphere had grown sentimental, William started glancing around as if he remembered something. He walked to the statue, raised his wand, "If I'm not mistaken... uh, left and right separate?"
"Click, click-click..."
With William's words, a subdued but not particularly apparent sound of stones moving arose. The next moment, under the watchful eyes of the three, the statue of the one-eyed witch slowly shifted, revealing a passage through which a person could squeeze through at its back.
"Is this... the secret passage to Honeydukes?"
Looking at the opening, a trace of nostalgic expression flashed in Lupin's eyes. He was silent for a moment, then firmly stated.
"Seems like it?"
William stroked his chin. He knew of this place from when he was in fifth grade. A Gryffindor student had told him about it to "borrow" a pack of dried Billywig Stings from Honeydukes' cellar.
He vaguely remembered that this passage could lead directly into the cellar of Honeydukes.
"...Is this what you guys have been studying all day in school?"
Behind them, Professor Meow spoke in a poignant tone, glaring at William as if slightly angry over his lack of focus — from her perspective, William had been in school for less than a year and a half, and had quickly found his way to the castle's secret passages...
Clearly, this was a sign of "a mind that's not quite set on learning."
"...Should we go in and take a look?"
Faced with accusations from the deputy headmistress, William paused momentarily, then awkwardly shifted the topic.
"Hmm, but I should inform the principal first."
Hesitating briefly, Professor McGonagall nodded, then waved her wand, and the armor at the far left of the hallway started to move. It wobbled towards Professor McGonagall, and after receiving instructions, continued its shaky, clunky path towards the stairs.
"Alright, we need to move fast —"
Professor McGonagall was the first to enter the narrow secret passage, forcing her to bend as she advanced, "It's nearly midnight, time waits for no one."
...
The journey to Honeydukes was uneventful, but the narrow passage had undergone some changes since William's last visit. After clearing the originally spider-filled way, the subsequent disturbances had led to some collapses, making the path increasingly winding and twisted —
Even though the trio's pace was brisk, they still spent almost half an hour reaching Honeydukes' cellar.
Moonlight filtered through misty glass and spread across the floor, the daytime rain finally abated. Near midnight, the candy shop was quiet except for the occasional crackling from the Exploding Bonbons on the counter.
"Not bad taste."
William grabbed a handful of Fizzing Whizzbees and, perhaps because of the added Billywig Sting, began to float gently.