It could be the proximity sensor, if the roundabout is small/enclosed by bushes or has chevron boards next to the curb, etc. but you need to be within a few feet of the obstacle to activate the alarm. I drove a Mercedes once which kep getting frightened by one particular bend (chevrons) even though it was 20 yards away. You can test it by disabling the park assist (button by the lower USB ports, ahead of the gear lever).
Other alarms, like low fuel, low brake fluid, or low engine oil ((if the liquid in queston sloshes to one side and makes the sensor think you're running out) would leave a warning on the dash for a few seconds... so I don't think it's this.
It may also be something weird like a heavy bag on the seat... the extra cornering force might fool the car into thinking someone is in the seat/not wearing a seat belt. If you have anything on the seats (even a child seat) then that could be possible.... althouh you can eliminate it by doing up all the seat-belts, even if nothing is on the seats.
If you're careful not to crash the car, it'd be interesting to have a passenger with you, to watch the dashboard for any error messages that you might not see behind the steering wheel spokes before they disappear.
Ralf S.