Secret Correspondence

Exploration Stage: Octopus Citadel
From: Mr. Black
Description: I followed Violet - she just returned to the Security Service headquarters through the back door. Before going to her office, she read a note, then set it on fire, and threw it into a trash can. I managed to put out the sheet, but it was carried away by a draft. Help me find it.
Completed: Good thing you noticed the burned note between the floorboards! Let’s see why Violet got rid of it immediately after reading. Fog take me, the whole text is burned off...! But a mark in the form of a snake remains. The sign of the Cult? So, it was a message from them!