Trimorph
Leave the red-lit lab through the double doors. The Trimorph appears in the lit metal passage past the autosave point.
Abomination
In Central Security, look for a keypad door with a triangular emblem. An Abomination can appear near the door and in a separate mesh-lined passage. As you approach the one near the door, look for the prompt to crouch and sneak up with your wrench.
Maintenance Drone
Near the end of Central Security, look for a walkway beside red-topped equipment. You may encounter the Maintenance Drone here around the time the SYS.NERV reboot fails and red beams cross the walkway.