Today's announcement by Citigroup that it has become a...

Today's announcement by Citigroup that it has become a substantial holder seemed straightforward. That is until you read that the date of acquisition and consideration were "N/A - Unknown to Citi".
Does anyone understand this?
As a second point, the substantial holder notice of the 12th Oct has Citicorp (again) nominees holding the particular shares for Evolution Master Fund et al registered in Georgetown in the Cayman Islands.
All this may mean nothing at all, but as a holder I would like to know much more. With the reasonably high trading volume at present my presumption is that something is happening. But what?