When I was in the Marines, we were taught to sweep with our eyes, and basically point our rifle wherever our eyes go. This is not outside the realm of good sense in the case of training for what we may encounter in combat. Much of the time we were going into urban environments where engagements can become almost comparable to whack-a-mole where you may get engaged from one window to the next by multiple threats firing and moving. In this case, i felt like the threat scan had valid uses in its application. Once you take fire from one window or doorway, you want to get the upper-hand the next time around, so you want to cover as many of the others with your weapon after responding initially. But the difference is that we were legitimately LOOKING and scanning with attention to detail. The stuff I see today with the fast side to side swinging is almost comical. In this discussion, I will just focus on the threat scans for those who stubbornly wish to apply it to their practice routine.
If you are going to apply a threat scan with your carry practice routine, I recommend using a more organic method that is a bit more realistic and applicable to all cases. After responding to a threat and using deadly force, your first priority should be cover. If you are already behind cover and your threat is down, this would be the time to quickly check your surroundings. Basically a quick but detailed look around will do it. But on big long scan shouldn't be a method you use. I prefer a more natural approach where you don't have to learn to scan, since most of you already know how just from doing day to day stuff. Just like when driving and looking at your surroundings, afford yourself a second or two to check around you but refocus on your threat. This should be broken up in segments. Scan in the directions you can cover in a comfortable amount of time, while realizing that your threat could start moving at any time. Remember to check your surroundings often, just to be aware of everything going on around you after using deadly force.
Now developing a standard of where to look and which direction to sweep should be avoided at all costs. Of course being able to see everywhere all the time isn't realistic, so you have to learn to apply good tactics and methods to the situation at hand. Wherever you are practicing, scan wherever you feel you need to scan. The first area that you feel has the most potential of activity that you need to be concerned with could be anywhere when the time strikes. Would it make sense to do the 360 degree turn around, like you see some people do, if your back is against a wall? Or would it make sense to stand still while looking side to side quickly if all that is on each side of you is walls? My point is that you need to keep a head on your shoulders when you are training to fight for your life and for after the fight. You are responsible for your own security, and you need to treat every practice session like its the real thing if you really feel that you need to incorporate the threat scan into your routine.
Without writing a novel about how to handle every conceivable situation and defend yourself 360 degrees anywhere, my advice is to keep it simple and very natural. Seek cover, maintain a low profile, cover your target, but keep an eye on your surroundings as you do when you are driving. Don't break concentration from your threat, but don't forget to be on the look out for the situation around you. As they preached in the Marines, "security is 360 degrees all the time, no excuses". Look where you need to and watch what is going on around you.