Upvote:3
Airlines can know you are in the airport if you checked your bags in. It is illegal for the plane to leave with your bags if you're not on the flight. At security, I assume they don't really care about your booking, only that you are on a flight and have the authorisation to be in that area of the airport.
On all international flights, and on domestic ones in most of the world, airlines adhere to a policy called Positive Passenger Bag Match (PPBM), which states that bags flying must be matched to a passenger on-board the flight.
Budget airlines won't always do final calls, sometimes they just shut the gate 20 or so minutes before the flight.
This can vary from one airport to another, some airports don't even do final calls anymore as seen here:
So they don't always know you are at the airport if you haven't checked in your bag.