Move fast enough between two points, even if in full view of the enemy, and the chances are that the only response you’ll get is a quick “Huh? What was that?”, before the guard in question goes back about their business. A lot of the time you’ll be flat-out ignored. At worst, they’ll think you’re the wind. But let’s look at the science of this, shall we?
The shortest period of time that a human brain needs to register a visual stimulous is 16 milliseconds. That’s 1/1000 of a second. It’s pretty short. Now let’s charitably assume that the distance between the two points of cover you’re darting between is only three metres. Let’s charitably assume that a decrease in the duration of your exposure of just one millisecond, to 15 milliseconds, is enough to render you completely invisible. Using the equation of Speed = Distance/Time, we can calculate that in order to travel the required 0.003 km in a time of 0.015 seconds, you’d have to be moving at 0.2 km per second. Scale that up, and you’d be running at 720 km/h.