How was someone else watching my Arena battle live?

During a recent primary Arena battle (7), the eyeball icon appeared, indicating that someone was watching; then confetti flew indicating the observer was cheering.

As this was a battle in the main arena and not a "friendly" clan or "tournament" battle, how did a third party get the ability to locate and watch my battle live?

Both myself and my opponent were in clans; I can only assume, but have not yet had the opportunity to prove, that if you go to your clan member list, and you catch any members currently in battles, you can observe them.

Clash Royale said that there is a new feature allowing players to spectate their friend's battle:

Spectator: See when friends are online and spectate their battles

So it was either your friend or your opponent's friend that was spectating the ongoing battle at that time. By "friend", I am referring to a Facebook friend added through the Clash Royale app.

Update A change in March 2017 allows members of the same clan to watch other member battles live, similar to observing tournament battles.