At any rate, the direction in which explanation is to be sought is clear, and the jewel-case points to it.

The real murderer dropped those ear-rings.
The murderer was upstairs, locked in, when Koch and Pestryakov knocked at the door.

Koch, like an ass, did not stay at the door; so the murderer popped out and ran down, too; for he had no other way of escape.

He hid from Koch, Pestryakov and the porter in the flat when Nikolay and Dmitri had just run out of it.

He stopped there while the porter and others were going upstairs, waited till they were out of hearing, and then went calmly downstairs at the very minute when Dmitri and Nikolay ran out into the street and there was no one in the entry; possibly he was seen, but not noticed.