Cordless Mice are quite adept at wandering off under their own steam. I have used half a dozen in the last few years, but as was said earlier, changing the batteries is a good first step. If youknow your batts to be good, then re synchronise the mouse with the rcvr. That should sort things out.
Conan
PS Guess what I am just doing? Yup - installing a new mouse and keyboard... My Belkin mouse was playing up and they sent out a later model mouse and a new and better keyboard too. Belkin apparently have a lifetime guarantee on many of their products and the customer support is excellent. Ten out of Ten, Belkin!