According to the trueachievements app, it's theone, but it still appears as "secret achievement" on my console. I obviously fullfilled the requirement, since it's plot-related, and I actually completed 100% of the game anyway so I have all other achievements but that one."find a chronon harness for Beth"
I played Act 3 part 1 four or five times already, even restarted a game from scratch , but it didn't make any difference.
The achievement just randomly (or maybe not so randomly) popped up this morning, yay ! ^^