Is it just one beep or the three beeps on startup?
It has been suggested on here that this could be down to brake fluid level being low, likely due to low break wear or possibly a faulty sensor that reports the brake fluid. But in those cases the handbrake light is said to flash (if the handbrake is off).
Try searching for three beeps on startup and it will return a series of posts that may be useful.
I get the three beeps very very occasionally, to the point where it doesn't bother me but I know my brakes need replacing, discs and pads on the front but I am hoping to sort that before the week is out. So fingers crossed that will be the end of it for me!?