Twins

Head to the Museum Infinitum and during a Ravager interact with the display right of the big window in front of the rocket contraption. Afterward, flip the round and listen near all of the displays in the museum for one that is playing creepy/haunted sounds and interact with it. Do this two more times on the following rounds and the twins will appear somewhere on the map for you to find and walk up to and obtain your rewards.

The twins will appear every couple of rounds randomly throughout the game from here on out and can be a good source of loot. The first time you interact with them, you will always be given a free Cryo Freeze ammo mod.