A booked seat is held for fifteen minutes. After that the booking lapses and anyone may sit there, whatever the small screen on the desk still shows. The system cannot tell whether you are in the building. It knows only whether you have tapped your card on the reader at the desk itself. A seat that has been booked but not tapped is an empty seat as far as everybody else is concerned, and staff will not ask a reader to move on the strength of a booking. A booking runs for three hours and cannot be extended at the desk. If you mean to stay longer, make the second booking from your phone before the first one ends, and tap again when it does.