The police officer mentioned two witnesses, the train conductor and a 73 year old woman with degenerative eye issues. Take a look at the grabs john4200 posted. I think it would be difficult for the camera to see Underwood at all. He picked that location for a reason. He may be on a camera in the station (hence the coat, hat and glasses) but not on that camera, talking to Zoe. As for the strange behavior, part of this depends on the resolution of the camera. Generally speaking, these cameras aren't high resolution so details such as Zoe's lips moving may not be distinguishable. (Plus a police officer reviewing the recording may simply be fast forwarding through the section before her death and not see details.) So they saw her playing with her phone, maybe talking a bit to herself and walk around the fence and off camera. On a CSI type show, I'm sure they would zoom in on a reflection on the fence post, enhance it to show a man, then digitally enhance the face to remove the hat and glasses to reveal Frank Underwood. I don't think that's going to be the case here.