This is a guide for the missable achievements Forty-niner and Responsible Citizen.
Both of these heroic feats depend on the 3rd in-game chapter, so you should start your most excellent adventure there by loading said chapter:


Once the level loads, you should follow the natural progression of the story up to the point where you clear the deadly gas from the room through the password protected console.

After the gas gets vented out of the room and you're done talking to your teammates, you should exit your armor suit and have a look around the room.

Through some miraculous act of God there's a total of 5 hidden gold coin pickups in this room and you should gather them all. I'll be using the entrance as a reference point.

First pickup worth of 3 coins is located near the entrance. Turn right and face the pool filled with deadly gas. The coins will be hidden behind the pipes.


Second pickup worth of 1 coin is also near the entrance, right under the floor panels and between the exposed pipes. Exact location shown below.


Third pickup worth of 1 coin is located in the corridor before the console you're supposed to use to vent out the gas. The coin rests on top of one of the big monitors on the left side.


Fourth pickup worth of 3 coins is located behind the console you're supposed to use to vent out the gas. The coins are tucked under a cable.


Fifth and final pickup worth of 1 coin is located on the left side of the room. From the entrance turn left, approach the broken monitor with the console password written on it, look below and you should see the coin sitting safely on top of the exposed wiring.

For this Herculean labor you'll have to examine the corpse in the room and pickup the "phone" pictured below.


You should continue on to the next chapter and there will come a time where you'll have the chance to use the "phone" on the elevator control terminal. You shouldn't. Instead, you should claim ignorance like so:


Eventually, the Spanish inquisition will barge in the elevator and you'll reach a point where you can pick a person to talk to. You should pick the cool dude and you'll have the option to report to him that you've found a dead body as shown below. What you pick afterwards is of little consequence.

You have reached the end of this fascinating guide and for that you deserve a standing ovation.