Hrm! So, after Petra tells you about the intruder, you can either tell her to watch the cameras to try to narrow down where he'll be, or you can tell her to go to whichever location you think he'll be at, and lay a trap. If the first place you tell her to go ("Watch the gym") happens to be where the intruder turns up, then that should trigger the achievement. If you're wrong (he was actually at the Office that night, instead), then it's not your first try anyone, and you don't get the achievement.
Does that match up with what you did? The very first time you actually told her to go watch a specific location, he happened to be at that location that night?