the doors one
The first three are after entering the first giant vault door. When you must use the elevator to launch yourself, instead put the portal under the slant and X button all three doors. Then, when you get to the control room with GLaDOS, there's a room just above it. Behind some equipment is a silver sliding door. Jump over the equipment and it will open, Examine the three doors in there for the achievement. Also, the missing experiment is back there.