This doesn’t explain your girlfriend saying that she saw it, but if she just said she did because she wanted to go back to sleep (which seems consistent with her going right back to sleep and being grouchy about being woken up), perhaps this. Apparently, the same sort of phenomenon that produces lucid dreaming, being on the edge of sleep and wakefulness, can also produce hallucinations. These are usually visual, and seeing lines has apparently been reported.

https://en.wikipedia.org/wiki/Hypnagogia

Hypnagogia is the transitional state from wakefulness to sleep, also defined as the waning state of consciousness during the onset of sleep. Its opposite state is described as hypnopompia – the transitional state from sleep into wakefulness. Mental phenomena that may occur during this “threshold consciousness” phase include hypnagogic hallucinations, lucid dreaming, and sleep paralysis.

Signs and symptoms

Transition to and from sleep may be attended by a wide variety of sensory experiences. These can occur in any modality, individually or combined, and range from the vague and barely perceptible to vivid hallucinations.

Sights

Among the more commonly reported, and more thoroughly researched, sensory features of hypnagogia are phosphenes which can manifest as seemingly random speckles, lines or geometrical patterns, including form constants, or as figurative (representational) images. They may be monochromatic or richly coloured, still or moving, flat or three-dimensional (offering an impression of perspective). Imagery representing movement through tunnels of light is also reported. Individual images are typically fleeting and given to very rapid changes. They are said to differ from dreams proper in that hypnagogic imagery is usually static and lacking in narrative content, although others understand the state rather as a gradual transition from hypnagogia to fragmentary dreams, i.e., from simple Eigenlicht to whole imagined scenes.