It wasn't that far, where your route left the public areas. Conveniently, you found the door to the maintenance stairwell open. It was also obviously forced, broken plastic and parts of the locking mechanism lay scattered on the ground. Inside, the air was a few degrees warmer, and instead of the ads, you heard distant voices of heavy machinery, both from above and below.
The lighting here proved trickier than outside. The stairwell was sparsely lit, with a single lamp on each level, and it probably got less power as it only shed minimal light. Pools of darkness welled in the corners, besieging patches of partial light.
According to the rout, you had to go down.