I worked as a lifeguard for a few summers in high school and college, mostly as a supervisor in the latter years. You're trained to develop a scanning pattern,

I worked as a lifeguard for a few summers in high school and college, mostly as a supervisor in the latter years. You're trained to develop a scanning pattern, and to not overly fixate on potential problem cases. Otherwise your tunnel vision might cause to miss something important that you hadn't noticed beforehand. I found that I got a pretty good intuition for things not looking "right".