In some of the more techonologically advanced rest rooms, there is no way for you to flush. The toilet has an electronic sensor which is supposed to flush automatically when it senses that somebody has come and gone. I have restrooms where there is no manual back-up.
In my experience, sometimes it flushes twice--one when you rise from the seat, and again when you open the stall door and leave. Unfortunately, sometimes the sensor is not working, and doesn't flush at all.
Note to building engineers: Always have a manual flush available for the user; no electronic device always functions.