winter driving engine stays on

I have a 2011 sonata and was able to get 38 city 41+ hiway during the summer.

it is at 23mpg now and mostly due to fact the engine stays on all the time. battery charged over half, stopping at a light, heat is off, blue drive and engine still runs. (so it shows instant mpg of zero while stopped)

Is that normal? pls advise if there is a certain temp that requires engine to stay on.

or should I go to a dealer?