I thought it might be in response to me looking at her. But once I was looking down concentrating on getting my coffee from the machine when I noticed from the corner of my eye she walked in the room while talking on her phone and stopped across from me, looking at me, but talking to whoever, so I turned my head upwards to meet her gaze, and held it for about 3 seconds before walking away with my coffee. Her eyes didn't move when I met her gaze, because she was already looking directly at me (as I suspected).
I make small talk with her sometimes, she is friendly and polite but not all that enthusiastic to talk to me it seems.