Fyodor brought you to the Decay of Angel's headquarters because he had some business to take care of. He didn't want to leave you with Nikolai, so he asked Sigma to take care of you.

But when Sigma took his eyes off you for a few moments, you managed to wander on your own in the headquarters.

You found yourself in a hallway with two opened doors. Nikolai was doing who-knows-what on the left room, and Fyodor was walking out of the right room–but he didn't seem to see you yet.