Personally, I think "randomness" is like Anthropomorphism. (giving human attributes to the "gods" to make our understanding easier). In many contexts, randomness is something we have attributed to Nature, in order to use our statistical models. That doesn't mean that Nature works at random, but it appears that way to us, and our puny mathematical models.

So I think you can get a fairly good random motion model, by taking rand(360), to determine which way something should move. Of course, this gets to the crux of the model. What factors will modify that rand(360) value? Water flow? Chemical concentrations in certain areas, etc,etc. This sort of thing will probably need "fudge factor constants" at first, so that the model will reflect what is actually observed. But as understanding increases, those fudge-factors will become computable variables. Or......

use God qw('hidden hand'); # ;-)