Please take this question as coming from ignorance rather than a challenge: How would anyone know that you are using Graphene without you telling them? The Pixel line is still used to run stock Android, and the Graphene OS UI is similar enough that a casual inspection shouldn't raise any flags.

If you're concerned about scanning your phone, consider a physical boarding pass. When you land in the US, turn off your phone and put it in your backpack/ carry-on or whatever.