After finding the 2 Raptors I started to fight them, but one of them was accidentally killed by a bot that was shooting me - and apparently the mission doesn't count that as a kill that advances the mission. I killed the other one myself, the timer run down to 00:00.000 and the mission didn't end. I had to abort it and take it again.

By the way, the 15-minutes limit may be too short for players with slower loading times; if I hadn't been using a Valk, I would not have made it, as the Raptors were in one of the last few sectors I checked and I had around 2 minutes left.