I was under the impression that at least prior to book five, Voldemort was in no state to go waltzing in anywhere and least of all Hogwarts because he considered Dumbledore to be the only wizard who could match him. Before the last book, Voldemort always has his actions carried out by his own agents.

As for the fireplace thing itself, I don't know, I just figured that Sirius had found someway to distract the surveillance or something or that they simply keeping only certain fireplaces under watch, discounting the Hogsmeade house because it didn't belong to Black. No idea how to explain the lack of watch on the Gryffindor fireplace though.