That's normally an indication that there is activity on the data connection. Something is uploading or downloading; OS update, iCloud backup, carrier network settings update, etc.
It's a known bug (there's a huge thread about it over on the official Apple forums). If I leave it alone it will sit there spinning for days eating the battery. I have to go into Settings and reset my connection to make it stop. It'll go away for a few weeks, then come back.