After proceeding up the stairs in the southeast of the level, there is a tall, rusted, metal panel in the southern wall. Press it to find a secret elevator. (sector 39)
Use secret #2 to access a secret corridor. (sector 40) At the east end of the corridor is a box of ammo.
The space immediately southwest of the opening you enter secret #3 from counts as a secret. (sector 41)
Jump across the gap to the west end of the corridor from secret #4 to register another secret. (sector 44)
Finally, jump down the hole at the west end of the corridor from secret #5, and you will land on a secret sector in the first room of the level. (sector 45) Note that due to its location, it is possible to access this secret before any other secret in the level, even though you are meant to get here from secret #5.