Way I see it; Defiance dropped of the grid, but her commander (Erich) would try to contact her if she were still in line of sight to an allied unit; now that the company's (mostly) extracted from the mission, Defiance has slipped off into the night to reappear whenever you see fit. That's it, that's all, no more mention of the Black Knight, because, as far as I'm concerned, she's decided to stay on-planet and didn't bother telling anyone until she had an opportune chance to slip away while the rest of the assault lance was tied up either babysitting Varyag, or playing 'Hammer the Pillager into dust'