The disguised man is Lupin.

Directly the lift descends and the doors open, throw yourself on him! Hold him! Shout for assistance!" He almost bellowed the last words into the policeman's ear.
"Yes, yes," said the policeman.

And he braced himself before the doors of the lift-well, gazing at them with harried eyes, as if he expected them to bite him.
"Be brave! Be ready to die in the discharge of your duty!" bellowed Lupin; and he walked out of the room, shut the door, and turned the key.
The policeman stood listening to the noise of the struggle in the lift, himself strung up to fighting point; he was panting.

Lupin's instructions were whirling and dancing in his head.
Lupin went quietly down the stairs.

Victoire and Sonia saw him coming.
Victoire rose; and as he came to the bottom of the stairs Sonia stepped forward and said in an anxious, pleading voice: "Oh, M.Guerchard, where is he ?" "He's here," said Lupin, in his natural voice.
Sonia sprang to him with outstretched arms.
"It's you! It IS you!" she cried.
"Just look how like him I am!" said Lupin, laughing triumphantly.