Looking at the intricate magical structure inside the box and the troll that seemed to be sleeping soundly, Avada was at a loss for words.
Voldemort... He really wanted to drag Avada onto his ship for good.
Avada still remembered that, during Halloween, Quirrell would release a troll as a diversion while he tried to sneak into the Third Floor Corridor and steal the Philosopher's Stone for the first time. But Snape, who had been on guard, stopped him, so Quirrell had to pretend nothing had happened and follow Professor McGonagall and the others to the scene.
That time, without an accomplice, he could only release the troll and then run off to report it in a panic. Then, when he tried to slip away from the teachers and sneak into the Third Floor Corridor, the flaw in his plan had been far too obvious.
With Avada's help this time, all Quirrell had to do was sit calmly in his seat and attend the feast. Then, after the troll's roar rang through the castle and everyone fell into chaos, he could run to the Third Floor Corridor in a "blind panic"...
After seeing the magic effects on the box—"Powerful Protection," "Remote Activation," "Self-Destruct," "Agitate Troll," and "Monitor Surroundings"—as well as the Sonorus Charm on the troll, Avada had a pretty good idea what Quirrell was up to.
Although this trip was doomed to be fruitless—first, the Philosopher's Stone wasn't even in the corridor room right now, and second, Quirrell didn't know how to deal with Fluffy, so he couldn't sneak in silently... Still, Avada would now have something Voldemort could hold over him. At least, that was what Voldemort believed.
Avada guessed that, according to Voldemort's plan, he would panic upon learning that he'd been the one to release the