Tunnel Rat gave you this spare rebreather device, saying that, 'we'll all need this when the toxins from the tunnels are up in the air'. It reminds you of the time you worked with her, an experience you call...
Getting Out Alive
It began when you learned that a PPD officer, one Paolo Marino, wanted help in getting his family out of Praetoria. You located his father, a respected doctor within the city, and 'kidnapped' him to get him into the hands of the Resistance.
You and Tunnel Rat discovered afterwards that Paolo's brother was in the Syndicate. You went to Syfotine Industries and destroyed the computer that tracks all members of the Syndicate, allowing Paolo's brother to freely make contact with the Resistance.
Complications arose though; you discovered in the Syndicate base that they had the location of every single person the Resistance ever helped out of the city! The Syndicate planned on using this information to blackmail the Resistance into doing whatever they wanted. Tunnel Rat also found out that Paolo Marino was put in charge by the PPD to track down all the people that the Resistance had taken out of the city.
You attacked the Syndicate and recovered the list, saving all those people currently in hiding. You were faced with a decision within that base after confronting Paolo Marino - leave him to his work in the PPD, or force him to be with his family, despite the risks.
Tunnel Rat, however, did not care at all about the choice you made - she was just happy that things were finally calm, saying that she wasn't one for all these complicated problems that came up.