What's recorded here is accurate but incomplete. It doesn't identify why Navaro's audio log talks about a tunnel back to the Farmer's Market. I've looked all around Hephaestus and haven't found anything, then went back to the Farmer's Market and though I found a crawlspace or two I missed the first two times I didn't locate any type of tunnel, especially one with a code entry device.
I suspect this is an incomplete item like the HK-47 factory in KOTOR: The Sith Lords
There is a locked room with some goodies in the farmer's market. That's probably it (I hacked it, but it was the only 4 digit code lock on the level).
Maintenance code[]
This code is for the pump room in the Farmer's Market. A room containing several items of interest, but by this point I'm sure most of the people trying to access this room had already hacked their way in.
Hmm, thats why there were the extra Auto Hack Tools in that other sercet compartment (above one of the turrets) - usually they have some laying about when something significant to hack into (and in hard the player might not have alot of resources to make.buy them)
Thats also a long crawl to Farmers market. But there probably were alot of maintenance ways (sewer system Atlas favored etc...) required for city utilities - they probably used little golf carts to get around like they do underground in Disneyland.