How to find the 3 Secret areas in Doom level 1, Hangar:

(Click here for a map of the score-affecting Secret areas.)

When you first appear on this level, you'll be at the south
end of a room, and you'll be facing north.
Walk to the northeast corner of this room, and enter the
hallway there.  Follow it to a door, and go through.
Go through the next room, to its southeast corner, and step
into the short, curved hallway there.
Follow this hallway to a large room, where a zig-zagging
walkway leads south through a wide pool of green slime.
Go south, to the far end of this room, where you'll see a
door at the end of a straight hallway.
Just before you get to that door, search the right-hand
side of the hallway for a secret panel.  When Activated, it
opens like a door.

Secret 1: Walk through the open panel, through the tunnel,
and out into the large courtyard beyond to get credit for
finding this Secret.  There is a blue Mega-Armor vest out
here, which you can pick up if you wish.

Go back indoors via the tunnel.  When you step back out of
the tunnel, turn right (south) and open the nearby door.
Step through this door, and walk to the middle of the room.
By doing so, you'll be crossing a floor trigger that lowers
a platform/barrier in the room behind you to the north.
Turn around and go back through the door, and walk north,
to the middle of the zig-zagging walkway in the slime.
Turn to face southeast and you'll see a newly-opened alcove
in the corner of the room.  (This alcove was previously
blocked by the platform/barrier that you just lowered.)
Run across the slime and into the alcove.

Secret 2: Once in the alcove, look to your right for a
shotgun in the southwest corner.  Picking it up counts as a
Secret discovered.

There is a hidden elevator in the southeast corner of this
alcove, but it cannot be activated here.  (The elevator is
controlled by another floor trigger.)
Turn and leave this alcove, running back to the middle of
the zig-zag walkway.
Go north, and into the curving tunnel beyond the zig-zag
walkway.  As you do so, you'll cross the floor trigger that
controls the elevator mentioned above.  When triggered, the
elevator drops down for a few seconds, then rises again.
(Wait a few seconds in the curved tunnel for the elevator
to cycle through one time.)
Run south across the floor trigger to drop the elevator,
and continue running southeast into the alcove.  Look for
the lowered elevator ahead, in the back left corner of the
alcove, and get on before it rises again.

Secret 3: Step onto the elevator and ride up.  At the top,
you'll find yourself at one end of a tunnel.  Simply walk
through to the other end of the tunnel, where you can jump
back down to the zig-zag walkway.