Loud buzzer sets off for no apparent reason two or three times instantly

Hello, You are right to look for warning lights when you hear the noise, and I would expect to see something about traction control or antilock brakes when hearing a warning sound on braking and turning. I do think the sound may be coming from outside of the vehicle, particularly the brakes. You may be hearing harmonic vibrations from the front rotors, or the brake pads could be wearing down. The Toyota Sienna has a long history of wearing out brakes quickly - like 20,000 -40,000 miles quickly, so try not to wait too long to have it looked at. If you start to notice the brake pedal pulsing under your foot or the steering wheel vibrating when you slow the vehicle, that is a sign of brake rotor warping, and this is also common on the Sienna. I would not go so far to say the vehicle is unsafe to drive or anything like that, just that you should have the brakes checked why you can. If you start to hear a squeal that stops when you press the brakes, it is time to have them changed.