Location: In the shipwreck. You have to play this as part of the story- literally can't miss it.
You flashback to the boat just before the crash.
Chat with your wounded partner, then start hunting down Eveline (no spoiler to say that you do not catch and neutralize her before she becomes a problem). You have to explore this floor before Mia will use the elevator, but there are no enemies yet, so take you time. In particular, there's a lot of machine gun ammo spread around here. Scoop it all up before narrowing in on your objective, which is just past the dining room. The double doors here are locked, but there's an opening the corner here, left of the doors and immediately left of what looks like a server rack.
Go through to encounter a pile of 'vomit,' and have mia recalibrate her tracker. You can also grab a Herb from the sink here- seems perfectly sanitary. There will now be a Moldy shambling around. There's a lot of ammmo in this flashback, so as long as you don't go crazy with it, feel free to execute any enemies you come across. As ever, retreat slowly, use aim, and with the machine gun fire in short bursts to minimize waste (while maximizing wastage).
Head to the elevator to catch a glimpse of Eveline. Because you can totally trust your eyes at this point.
Wait for the elevator, step inside, then immediately step outside as a Moldy tries to ambush you. Gun the sucker down, then catch a ride to level S2- the only floor available. Sprint down this section. As you reach a room with a particularly giant pipe to your right, a shambler will attack from your left, but it's not even worth shooting.
Just grab the herb and then machine gun ammo on you left as you exit this section, then close the door behind you.
This next section has a Scyther who *is* worth shooting, so do that. Note also the double doors here- you can't get through them now, but they're important later.
Grab some smart bombs in an open cabinet at the end of this hall, then turn left and through the double doors. Ignore the platform and stairs in the middle of the room, and the Moulder and Skully making their way laboriously down them- just make for the far door and close it behind you. Your now in a hallway where Mia thinks she has Eveline cornered (insert hollow laugh), but the door is locked. Alan advises looking through your luggage, which is the farthest door in this corridor, left-hand side.
Smash the crate for supplies, then pry open the green barrier to your left to reveal a treasure trove of Corrisive- a new item type.
Essentially these are little bottles of acid you can use to melt locks. So you can open the door to Eveline now or, if you want to play it safe, double back to those double doors where the Scyther was. All enemies in your path have disappeared, and this room holds not only more corrosive, but more bombs, and more machinegun ammo. These are all good things.
Go back and corrode the door Eveline is hiding behind. Search this area for her, though it's clear she'll only be found if she wants to be found.
Time to go back the way you came. There are more enemies hanging about now, and this time the two in the staircase room can't be dodged. Retreat up the stairs, leaving a smartbomb to take out the walker, then shooting or dodging the crawler.
Press forward back to the elevator. That room with the big pipe where you dodged an ambush earlier has at least three enemies ready to spring at you- use the machine gun to plow the row.
Get to the elevator just in time to see it close in front of Evie. How familiar.
Take the elevator up, then pry open the safety doors when it stops. You're stuck between levels, but [F] will let you climb up to the second floor of the ship.
Go to the guest room (the room where this flashback began), to get a hint about Evie's location.
Go back down the hall, past the elevator into the bunk room. You can use a corrosive to claim two bombs from the lock box here.
Use one of them to take out the scyther skulking up ahead. There's another lockbox with even more bombs in the corner here, if you like.
Go back to the dining room, then enter the room adjacent- the one that had a mold-blocked door you couldn't enter before. Now this room has an open vent high on the right wall.
Scurry up quickly, because the door mold will start to spawn enemies the moment you step inside. Turn left into the the stairwell and travel up to 3F. This area should start to look familiar.
Just outside the video room, you'll run into Alan, who's not doing well.
Once that little episode is over, you collapse and wake up…. still in the flashback. Head past the body into a room with a laptop.
Interact with it for another blast from the past, then open the far door and head down the corridor into darkness.