Is the brake warning light coming on. If so just try toppping up a little.
I had this symptom a few weeks ago, first it was suggested that new pads had been installed and it had not been topped up!!
I then had the warnig light on occaisionaly, no apparent reason.
I then got the brake light, VSC off and the buzzer.
There are a few similar symptoms, does the brake pedal go down if whaen stationary yuor foot is on the brake
After a few attempts I ended up in Lexus Stockport and as noted on this site it was the Master Braake Cylinder. At Lexus circa £1,750 to be fixed.
Before you get that far I suggest you replace the brake fluid, if you ave not aleady done so and give them a good flushing out.
It turns out that my Dec 1998 model already had a second hand unit on and it was this that had gone.