... would be low brake fluid level:

Going round the roundabout throws the fluid outwards, which the sensor detects as "too low level".

As you say, quite common that people takes roundabouts at higher speed (or at way higher g loads) than "normal"; also, a roundabout can give a consistent 'g' for longer - hence the problem may not show up on "normal" curves in the road...

But there's also plenty of other faults (or just "wear" conditions) that could be shown up by such coernering 'g' forces...