To start this side mission, you need to go to the small encampment just outside of Liar’s Berg, which you come to when doing the Best Minion Ever story mission . One of the bandits here will drop an ECHO Recorder upon being killed. It seems the ECHO Recorder was Helena Pierce’s and Hammerlock wants you to find the rest of them. The first one is inside of the dumpster, which is locked. Melee the lock to break it, then snatch the ECHO.

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You shouldn't have too much trouble getting the ECHOs needed for this mission, although you will need to do a bit of platforming for the second one (right).

The second ECHO is located on top of the building that Hammerlock came out of when you were first introduced to him. It shouldn’t prove too difficult to get up onto the roof, where the ECHO is located in the ice object you can break. For the third, and final, ECHO, is located inside of a container on the side of the building, which will be flashing green. Once you’ve gotten all of the ECHOs, return to Hammerlock to finish the mission (you get a choice of one of two pistols as your reward).