Used to be that planes with less than a certain number of pax didn't need screening. At one time (mid 90's), there was a group of 2-3 gates at one end of a DFW concourse that was unsecured and used by a small set of commuter flights using small aircraft. I forget what the number was, but it was on the order of 15, 16 people.

I, too, recall when airlines left the flight deck open. I was on an Austrian transatlantic flight where the flight deck was open from push-back to gate arrival. Yet we (and all other flights) had armed escort from landing to the gate....