I'll say what I can before bed, I'm no techno geek, but Flight Data and Cockpit Voice Recorders, (only the uninformed call them black boxes, they are orange) record data pertinent to the flight. Cockpit door openings? Ooookayyyy... We have had many debates using FDR data on things as insignificant as when an airplane actually touched down, there is plenty of room for interpretation of the data, yeah, it is that exact, not. Each company had its own Standard Operating Procedures, I don't know what they were for American or United, but they didn't change that much between companies. Some aircraft didn't even have electric locks, we didn't. It wasn't SOP to lock the door after takeoff in most cases. We even had an open door policy at one time. I do know how the the hijackers used companies' SOP's to their advantage though, but I won't get into that.
Primary Surveillance Radar picks up everything, including geese, and trucks, in places like Winnipeg. Transponders are set to a code so the information of the flight is displayed on radar, i.e. flight number, type, speed, alititude. If the code is mis set, Air Traffic Control cannot even see the aircraft on Secondary Radar, it happens all the time. There are discreet codes that can be set for things like emergency, communication failure, unlawful interferance, etc. Only transponder information is picked up by Secondary Surveillance Radar so as to reduce the ground clutter recieved by Primary Radar.. I can't explain it all, maybe there is an Air Traffic Controller here that can.