Over the past few years, my US bound flights have departed from NRT, MEL, SYD, HKG and LHR.

At HKG (CX flight), there were tables set up at the entrance to the gate area where they were checking all bags. I had a bottle of water from the lounge which I had planned to drink on the plane. They wouldn't let me through with the bottle, drank it there.
At SYD (QF), they had random bag checks with security guards grabbing people as one of the three tables became free.
At MEL (QF), LHR (BA/AA) and NRT (AA), I don't recall anything additional at the gate and only the normal screening for that port at the main checkpoint.