I think canon suggests you’re either part of the Floo network or you’re not, meaning that if you were part of it, the Death Eaters could use it to get in, just as you could use it to get out.
Before the war, certainly. And even during, while Dumbledore was alive, Hogwarts was well enough protected that it probably wouldn’t have been a good idea for the Death Eaters to try and get in by Floo, even assuming that they could.
The Potter Wiki says that the Floo Network is governed by the Floo Network Authority at the Ministry of Magic, which probably keeps tabs on who’s connected where, so being connected to the Floo Network may be incompatible with trying to keep your location strictly secret.
There’s also the Floo.
I think canon suggests you’re either part of the Floo network or you’re not, meaning that if you were part of it, the Death Eaters could use it to get in, just as you could use it to get out.
The offices of Dumbledore and Snape and presumably the other Professors have Floo links.
Before the war, certainly. And even during, while Dumbledore was alive, Hogwarts was well enough protected that it probably wouldn’t have been a good idea for the Death Eaters to try and get in by Floo, even assuming that they could.
The Potter Wiki says that the Floo Network is governed by the Floo Network Authority at the Ministry of Magic, which probably keeps tabs on who’s connected where, so being connected to the Floo Network may be incompatible with trying to keep your location strictly secret.