Image: UbisoftOfficial screenshot from Ubisoft.
Didn't See That Coming! unlocks when you use a Security Control Center to shut off every camera and alarm at once. It is a stealth trophy, and the bases with a control center are where you get it.
1. Pick an FND base and scout it from range with your phone. 2. Find the Security Control Center building. 3. Sneak in and use the control panel. 4. All cameras and alarms go dark and the trophy pops.
Clear guards near the control center with takedowns first. After the systems are down, take your time with the rest of the base. Watch for soldiers who wander in, since they can still shout for help in person.