When ever I do that its cause I spaced out and didn't mean to stare off at someone but I tend to be spacey a lot..it could mean that she likes you or finds interest in you or was just simply zoning out. It can mean a lot of different things so try and remember what her expression was to help you further understand..
Not to sound sexist, but you can never tell what girls looks mean. all throughout school i had girls staring at me, some liked me and others were interested in what me and my friend were doing. Others just stared off in space and noticed they were staring at me when I turned my head.
For me it mean that I find you interesting an when I look at you an I turn It means that I like you but I want you to come up and say it first but keep in mind that every girl is different so just go up to her and see what happens?.