Glossary
Shoulder peek
In one line: Showing just a sliver of your body (the shoulder) to make the enemy react, without really exposing yourself.
What it means
A shoulder peek is a tiny movement: you step out just enough for the enemy to see your shoulder, then go back. You do not need to see them: you need them to react. If they shoot, you know they are there and where; if they have an Operator, you make them waste a shot.
It is a safe way to ask an angle "is anyone there?". It works less well against patient players who do not shoot at a shoulder.
An example
You suspect a defender with an Operator watching the corridor exit. You shoulder peek: they fire and just miss. Now you know where they are, and you have about a second and a half before they can shoot again.