It seems to me that they could just floo in and out of the castle. In The Order of the Phoenix and The Goblet of Fire Sirius floos in to the castle. Well, only his head is going in and out, but I think a ward that prevented people from flooing in and out would work heads as well as bodies. And, if Sirius (who is considered a serial killer) can floo in, the teachers would obviously be able to too. (and yes I know that Dumbledore trusts Sirius. But, I think that if he could floo in- it must be possible for everyone.
In The Prisoner of Azkaban, Snape calls Lupin through the floo- when he is interigating Harry about the Maurader's Map. Lupin comes fully through the floo, so it must be possible for at least inter-castle travel. I would say that the evidence points to the floo in and out of Hogwarts being unregulated and unrewarded.
I don't think they even monitor the floo network, because when Umbridge talked about how she was monitoring it she pointed it out especially, which is something she wouldn't do if it wasn't noteworthy or unusual.
Dispite the fact that they have all the anti-apperation/disapperation spells on the grounds and spells to stop people entering at speed, Hogwarts seems remarkably lax in other security measures. I mean how many times have Harry, Ron, and Hermione been wandering in the hallways after curfew? And Sirius can get into and out of the castle without detection. One would think that if invisibility cloaks and being an animagi cannot fool the Maurader's Map (a device designed by school age wizards), the teachers (especially Dumbledore) should be easily able to figure out a way to detect unauthorized personnel in the castle or in the hallways.
So I think it is very likely that teachers can just floo in and out of the castle to get to and from work. Since, I completely agree with you that it doesn't seem particularly likely that the castle is all of their permanent residence. (Snape gets annoyed enough at the students during normal school hours, I think he would go mental if he was required to live in the same building as them for nine months.)
Though there is also the point that in The Order of the Phoenix, when Professor Trelawney gets the boot she says that Hogwarts is her home. So maybe some of them live there permanently. But, I would say that teachers probably have quarters where they can stay the night. I would assume that they have to have a certain number of teachers at the castle at any given time, because it would be incredibly stupid and irresponsible to leave 600 teenagers in a castle alone (or pretty much alone) every night. There is also definite evidence that teachers are required to patrol the corridors at night.
To sum up, I would assume that most of them live there at minimum part time- and many live there permanently. Therefore, there must be a convent means of getting in and out (I think that floo powder is the most likely) or it would basically bar teachers of having a life and family of their own.