[Zigûr]

A good post from Huinesoron. Here are a few things I've observed.

Originally Posted by Huinesoron

It sounds like Shagrat's initial report was simply 'we caught a prisoner, here's what he had'. From back in 'The Choices of Master Samwise', we know he probably didn't send that message ahead:

`All right! But if I were you, I'd catch the big one that's loose, before you send in any report to Lugbúrz. It won't sound too pretty to say you've caught the kitten and let the cat escape.'

I suppose it would depend on whether Shagrat followed Gorbag's advice or ignored it. If he ignored it and sent a preliminary report to Barad-dûr, that might explain the conflicting information the scouts had later.

I feel like there was a point where we saw a Nazgul leave the Tower, but I think that was probably to do with Isengard instead?

On the early morning of the 6th of March a Ringwraith flew to Isengard in response to Pippin looking into the Orthanc-stone; another one had already gone on the night of the 5th to investigate why Saruman was being tardy with his reports. At this time Frodo and Sam were still at the Morannon. By the time Frodo was captured on the 13th, I think Sauron would have definitely known that Saruman had been defeated and the power of Isengard overthrown.
However, when Frodo and Sam left Cirith Ungol on the 15th, a Ringwraith arrived at the tower:
"Out of the black sky there came dropping like a bolt a winged shape, rending the clouds with a ghastly shriek."
This Nazgûl was already in the vicinity the day before Shagrat reached Barad-dûr, and it doesn't seem to have been en route to somewhere else (such as Isengard or the Pelennor) because the Orcs in the area knew it had taken charge:

'I'll give your name and number to the Nazgûl,' said the soldier lowering his voice to a hiss. 'One of them's in charge at the Tower now.'

In addition, Sam felt it outside the tower for a period of time:

I think this place is being watched. I can't explain it, but well: it feels to me as if one of those foul flying Riders was about, up in the blackness where he can't be seen.

This suggests to me that a Wraith was sent to Cirith Ungol deliberately in response to something prior to Shagrat's arrival at Barad-dûr, which happened on the 16th according to the unpublished time scheme. In fact, the time scheme points out (as far as I can interpret Professor Tolkien's handwriting) that on the 15th "News of escape of prisoners of Tower reaches Baraddûr (sic) almost as soon as news of their capture."
I can't explain the use of plural, but this suggests the following to me:
1. Frodo was captured.
2. News of this was sent to Barad-dûr
3. The Orcs quarrel; Shagrat leaves for Barad-dûr
4. I'm unsure of this part, but then possibly news of the capture reaches Barad-dûr. A Nazgûl flies to Cirith Ungol but arrives to find the guards dead and the prisoner gone.
5. Shagrat arrives at Barad-dûr, is interrogated and executed.

Perhaps I've gotten the wrong end of the stick somewhere, but it seems to me that information came to the Dark Tower before Shagrat's own arrival.

In any event, this is all very interesting, isn't it? I always find piecing the timeline together satisfying.

EDIT: It also occurs to me that even if Shagrat did not send an initial report, word may have come to the Dark Tower anyway; Shagrat didn't trust either Gorbag's company or his own. The Nazgûl at Cirith Ungol may have been watching in response to the Silent Watchers at Minas Morgul, I suppose. There's a lot to consider.