Microsoft apparently has been busy lately. As already announced by Neowin, Microsoft is planning to release several updates to Office 2003, 2007, and 2010 next week. In addition, these updates will address issues in Windows and other Microsoft software. There is another update available courtesy of Microsoft for Office 2013 in particular.
This update came in today via the Office Update functionality in Office 365 Home Premium. The dialog box simply states that "Updates are available. Would you like to apply the update for build 15.0.4128.1025 now?" This update came in on a Windows 7 machine, but it is assumed that this update applies to all preview versions of Office 2013 or Office 365, regardless of the version of the software or OS.
The dialog box gives no indication of the urgency of the update either. It asks to shutdown a particular program known as Microsoft Office Document Cache Sync Client Interface. This seems to be the only hint as to what the update is for.
After the update is completed, there is no change to the build number in Office. This may mean that the update is minor, with only tweaks to specific features of Office. Neowin has contacted Microsoft for more information on this update. The only other indication that this update is being deployed is found in this Microsoft Community thread.