Gents,
I don't believe this folder is only created by accessing it from a non-Windows device because of it behavior (although I use DB on Windows, Ubuntu and Android): if I have 'Chrome 15.exe' in a DB folder, and I upload 'Chrome 16.exe' and remove 'Chrome 15.exe', then the latter file is put into my cache folder with a '(deleted <GUID>)' extension. The cache folder is only visible when having unchecked 'Hide protected OS files', however, there is always a possibility to just jump into that folder by typing it in Explorer or via the 'Run' dialog.
So if you remove files then the old files are kept in the cache. Another example is when you manually select which folders to sync. On one computer I had - by default - all folders synced. Then for security reasons on this portable netbook I decided to uncheck syncing my Documents folder. In DB the Documents folder was gone, but *all* my Documents were still present in the cache!
Having the 'Hide protected OS files' option unchecked does not show this folder in CCleaner. That would also a bit weird IMHO, since CCleaner is able to find other folders that are hidden (e.g. TB/Chrome profiles in AppData which are by default hidden).
So it would be nice if CCleaner is able to find the DB cache folder itself. Perhaps by default unchecked but at least listed so I can easily enable cleaning the DB cache.
There is a snag though; the DB user is able to manually define the DB folder, as a result the .dropbox.cache moves accordingly. So it's not always in %APPDATA%\Dropbox. For example I have defined my dropbox folders on all computers on D:\Users\<USERNAME>\Dropbox. This location will be recorded somewhere but a quick scan through my registry did not reveal the location.
If there are still doubts if it is safe to remove the DB cache folder then please read this thread: http://forums.dropbox.com/topic.php?id=140. Note that several ppl announced their folder is over 100 *Gb* ...