You see them as you get closer, and it is based on your PE (Perception), so you will see them earlier with high PE, or later with low PE. Sometimes with PE 1, you see them too late if you run too fast to a random footlocker, and get a broken leg, so maybe PE1 guys should slow down as they approach.
And if you're locker-hunting a lot, get Traps skill to 50 and get the Light Step perk. It helps a lot. Carry Doctor's Bag (or Paramedic's Bag) to help correct broken limbs. Disarming Traps can be a nice little boost to XP as you hunt, if you have much higher Traps skill (max 150).