Where’d that Nazgûl come from?

[Alcuin]

According to the Tale of Years, Frodo and Sam escaped from Cirith Ungol on the morning of March 15, the same day as the Battle of the Pelennor Fields. If you work out the timing, they crashed the gate shortly before dawn, some two hours (or more) before Éowyn (& Merry) killed the Witch-king.

As they escaped, a flying Nazgûl swooped down from the clouds and perched on the wall next to the ruined gate.

Where’d that Nazgûl come from? Minas Morgul? Barad-dûr? Why wasn’t he at Minas Tirith?

The same day, there was an assault upon Lórien, the second of three assaults launched by Sauron from Dol Guldur. It would make sense to have a Nazgûl there, too, unless Sauron had some other powerful servant to send against Galadriel.

Does anyone have an accounting of all the Nazgûl at this point? You get the impression from Return of the King that the Nazgûl were all winging around Minas Tirith, but that clearly isn’t the case. I don’t recall seeing a discussion in War of the Ring or Sauron Defeated, and Reader's Companion doesn’t even mention any accounting from unpublished notes.