So apparently, if you hang around the home screen long enough, you keep getting the message about changing your automatic unit lock settings. Well, in addition to that, every time you dismiss that prompt, you get an extra Arena Orb...

I wasn't really paying attention until I realized I just used 8 orbs in the span of 5 hours (3 for Col. Dailies and 5 for Arena Dailies), so I don't know how often the prompt comes up. I'm trying to check that now.

EDIT: So I just sat around and checked, and it appears that the prompt isn't the cause. The prompt pops up after an hour of sitting at the Home screen, but it didn't give me an extra orb this time. Stay tuned.