Whenever my MacBook Air starts up from shutdown (or on restart), once the Finder has started, it prompts me for my iCloud password, like this:
As you can see there is no title or anything to identify the app or process requesting the password. However, through the use of a clever Python script (unfortunately I can't remember where I got it from) I was able to determine the process requesting the password:
If I click Cancel twice (it pops up again after the 1st Cancel) it doesn't reappear and there are no apparent issues. All my iCloud services (Photos, Music, Contacts, Calendars, Keychain) seem to sync and work just fine.
I used Activity Monitor to dump the processes open files and the only file of relevance I can see is
/usr/share/icu/icudt53l.dat although I have no idea what it is.
The problem only started happening about a month ago, and there's no obvious change that occurred on my machine that I can think of that might have caused this.
EDIT: obviously I tried entering my iCloud password, without success.