When you hear scraping noises signaling that 173's nearby.
Or when you blink and he suddenly appears in the room with you.
You're just walking down the dark hall with the creepy ambiance playing when you turn a corner and find SCP-173 right in front of you, very close to killing you. If you examine the scene closely, you'll see a broken air-duct where 173 escaped meaning that it could be anywhere in the building. 173 then kills two D-Classes and a guard while the lights flicker and when the lights finally die completely, it's gone with you as the only survivor. You are escorted to SCP-173's cell and everything's fine at first, until the doors start malfunctioning and the lights flicker.