08 Mar 2012 12 Mar 2012
This achievement is fairly simple. As you scan (left trigger) an occupied system (red one with a Reaper on top of it), the Reaper awareness will go up. Those red arrows that appear at the edges of the system indicate incoming Reapers, and when the red bar maxes out, that's where they'll show up. Simply park yourself next to a Reaper-free edge of the system (or near the mass relay) and spam your scanner (hopefully after finding everything in the system). As soon as the Reapers appear, escape; you can do this by either leaving the system or hopping onto the relay network.
Some advice about escaping Reapers in general: you accelerate better than they do and can be more maneuverable, but they're faster. You'll get a good head start, but if you're too far away from an escape point, they'll catch you every time.
If you DO find yourself in a race to the edge of the system, it is vital - absolutely vital - that you shout "Meep meep!" as you escape.
EDIT: Oh, and the Reapers stick around after you leave. Once you complete a mission, meaning, physically leave the ship, shoot some things, and get back on, they'll be gone when you return.