How does the cow seem to enjoy causing a problem for Sylvia every night?

The cow seems to enjoy wandering away every evening, just as if it knows that Sylvia will have to come looking.

There was hardly a night the summer through when the old cow could be found waiting at the pasture bars; on the contrary, it was her greatest pleasure to hide herself away among the huckleberry bushes, and though she wore a loud bell she had made the discovery that if one stood perfectly still it would not ring.