According to this thread posted by one of the developers on the Steam forums, the achievement for sector 3 is broken and will not unlock, along with
- Finale
- Find the Developers
- Tough Going Part 1
- Tough Going Part 2
- Fizzicks
EDIT: The achievements appear to be fixed. The achievement for Sector 3 can now be obtained simply by playing through the sector like with all of the other achievements. I tested and every one of the previously broken achievements can now be obtained.
Xbox360Achievements actually lists their descriptions now, and I've unlocked them on Steam as well, so here's the deal.
The only true secret achievement is:
Decisions, Decisions - Refuse to cooperate with Victor in the Interrogation
At one point, you're forced to sacrifice either Frank or Eddie. If you don't decide by not pressing anything, one of them will be killed anyway, and the achievement will unlock. Not sure if it's random, but he killed Eddie both times I refused to pick one.
All of the other secret achievements are pure story achievements. If you play the campaign, you will get them, and going into too much detail would just spoil the story (which is probably why they're hidden in the first place). These are:
- Cheque Please! - Survive the hit at the start of the campaign
- I'm Just Getting Started - Find and interrogate Swifty
- Escape the Brotherhood - Survive the interrogation
- Storm the Mansion - Take back your mansion
- One Flew Over... - Return to reality...?
- Sweet Revenge - Avenge your Aunt Sarah
- Allies in Strange Places - Escape the asylum with help from the inside
- Date Night - Protect Jenny
- Back in the Saddle - Rescue Jackie from the Iron Maiden
- Step into Hell... - Defeat Victor
The last one is also a story achievement, but for the Vendetta campaign (more specifically, for finishing it).
Should Have Called 555-2368… - Survive the battle against the Hell Beast
The phone number is a reference to Ghostbusters, by the way. And, just so you know, there's a Hell Beast at the end of Vendetta mode. I bet I just totally ruined the complex and involved story for you.
Best Answer
The "Secret Sector" is a location you can visit while following the quest to repair the broken Stasis Pod. It is home to a secret race, and their secret ship.
Here's a good video walkthrough of the events you'll need to encounter in order to get there. Having the Rock cruiser's Rock Plating makes part of this easier, but it's still a challenging set of events to trigger properly: