Where I used to Hunt in the early Fall there were many Quail, like you mentioned they would disappear in the winter.
In the warmer months and Fall they seemed to mostly hang out on slopes in the weeds, scattered brush. I always figured they headed someplace out of the wind in the winter, like the thick brush or thickets near Creeks or arroyos (dry Creeks). I have no proof. like I said, it was an assumption as I rarely went down into the thickets. They had to be somewhere and where I hunted there just weren't that many choices, nearly bald hills, weed covered slopes and thickets in the low spots.
One thing I did notice about our Quail is they were good at hiding and wouldn't usually flush until you were almost stepping on them. I mean they would explode into flight right under my feet, most always startled me some.