Rashi explains the Mishna to be discussing whether we need to be concerned about a hulda bringing hametz into a place that had already been checked. The Rambam seems to understand the case otherwise. According to him, this Mishna is the continuation of the first Mishna in the tractate, which obligates bedika (searching) only in places where hametz is normally brought. Here we are taught that we need not be concerned that perhaps an animal brought hametz into such a place.
The Gemara points out that the Mishna’s rule will only apply if we do not see the hulda running into the house with hametz. Were we to see the hulda doing that, we would, in fact, insist that the house be checked a second time, and we cannot assume that the hulda ate the hametz that he carried in.
The Gemara asks whether the requirement to carefully store hametz that is meant to be eaten on the morning of the 14th of Nisan so that a further bedika will not need to be done does not indicate that we are concerned that a hulda may move it around. In response to this question the Gemara quotes an interesting exchange between Abaye and Rava.
Abaye said: This is not difficult; this ruling is referring to the fourteenth of Nisan, whereas that ruling is referring to the thirteenth. The Gemara elaborates: On the thirteenth of Nisan, when bread is still found in every house, the marten does not conceal the leaven, and therefore there is no concern that perhaps the marten dragged the leaven elsewhere and concealed it. However, on the fourteenth of Nisan, when bread is not found in any of the houses, the marten hides the leaven.
Rava said in surprise: And is the marten a prophetess [v’khee hulda nevi’ah] that knows that now is the fourteenth of Nisan and no one will bake until the evening, and it leaves over bread and conceals it in its hole? Rather, Rava rejected Abaye’s answer and said: With regard to the leaven that one leaves after the search, he should place it in a concealed location, lest a marten take it before us and it will require searching after it. Only if one actually sees the marten take the leaven, is he required to search after it.
In his response to Abaye, Rava is using a play on words. There is, in fact, a prophetess in Tanakh by the name of Hulda ha-Nevi’ah – see II 22:14. More importantly, his argument seems to make so much sense that it is difficult to understand what Abaye was trying to say.
Rav Ya’akov Emden explains that according to Abaye the marten is sensitive to the fact that there is less food in the house on erev Pesah than there is under normal circumstances, and begins to hoard bread. This appears to be the explanation of the Jerusalem Talmud, as well.