At pro level, much of the game is about what information you can gain, and about choosing what to show and, more importantly, what you don't show (hide) and acting on non-triggers.
An example of a non-trigger is knowing that if I haven't seen a certain unit at time X, I know I'm safe to do Y. It is acting upon the information that something didn't happen.
To expand: I saw my opponent starting two gases at my 21 supply scout. When I scouted again at 47 supply, I saw no gas heavy units, so I can deduce the gas was used for better technology. This will allow me the opportunity to increase my worker count by Z before building army, or I could try and kill my opponent right there for his technological greed.
An example of a non-trigger is knowing that if I haven't seen a certain unit at time X, I know I'm safe to do Y. It is acting upon the information that something didn't happen.
To expand: I saw my opponent starting two gases at my 21 supply scout. When I scouted again at 47 supply, I saw no gas heavy units, so I can deduce the gas was used for better technology. This will allow me the opportunity to increase my worker count by Z before building army, or I could try and kill my opponent right there for his technological greed.