I Am Fish

This is on the 3rd level of the piranha. The hospital. You can skip checkpoint to the point after dropping down the desk before you meet nigel.

I was able to slip through the gap between the chair at the right and the wall where he didn't notice me. Taking the left after then right to the stairs. Down the double flight of stairs.

I think the path in next room (the dark corridors) sticking to the junctions was left, right, straight, left, left, right, right. I didn't use any of the vents in this room. You should see the waiting area with a lot of benches. Go through them and stick to the left to the exit near the window. Turn right and go to the blue door. Take the stairs down and the achievement should pop when you go In the vent.

Wondering how to get to this point in the 3rd piranha campaign? Check out my piranha fish playthrough -
As for this achievement video:

1. Get to the 3rd piranha mission.
2. Half way through, a doctor will start talking to you. Hide in the vent.
3. Quickly rush down the hall, turning left, then turn right to go through the blue door.
4. Go down the outside stairs.
5. Enter the building and take your second left.
6. Turn right, then go straight, then left, then left again, then right, then right again.
7. You'll know his a big seating area. Turn left to go through the middle of the seats as quick as you can.
8. Go diagonally to the back right, then through a blue door.
9. Immediately turn right to go down the stairs, and into the vent.