In Non-Stop we saw that the copilot was killed from a dart like thing shot from a hole in the toilet wall. But who killed him and how did he get killed?
The only explanation besides it being a hole in the movie, is that the poison darts have a very long delay time (20-25 minutes). This opens a large gap of time where the ex soldier could have used the toilet before the old lady, and shot the pilot.
The movie doesn't show who uses the toilet before the old lady, just after to make Jen a suspect.