As soon as you enter the building, go straight to the main doorway then turn right by the elevator. Look for the door with the “Toilettes” sign. Look for the poster on the floor of the second open stall.
Continue with the mission until you get the chance to sneak in the other building with weapon drawn and with the objectives to plant the explosives by the gates. On your way to the first gate, you’ll come across an optional objective to “Rescue Fischer”. You can do that first then get past the parked truck then pick the lock (open the door instantly if you have pickpocketed the key from an officer earlier) to reach the next building.
Go upstairs (there’s a lone guard patrolling the first and second floor hallways) then enter the first open room in front of the stairs. The bottle is found on a chest to the right.
Once the assault with your French resistance allies begin, you’ll have to clear the enemy position in the plaza in front of the church. After clearing it and before rendezvousing with Crowley, check out the crate in the middle of the plaza near the building to find this memento.