Hi, IMR and Kat are both right I am sure it is the clock foot battery, I had a problem with my clock foot battery . Within a minute of installing the clock foot battery, I broke off a prong, so be VERY careful. I had a hard time getting myne to work after that, but luckily there are some GREAT people on here and another member was able to help me get the wires crossed properly so my Pleo rb would work, I had just opened her, so it would have been terrible to not get her going.
If after putting the battery in as suggested and your pleo still is beeping, then I would guess it is a flat battery, I know I had to go get one to replace one that did not last long and it cost about $4.00 or so for the new one at a local drug store chain so at least It was easy to find. Good luck.
RWM