This happened back in 2012 or 2013 on one of the FB app releases. I remember bc a friend had just gotten their first iPhone, a 3GS, and the battery kept draining. Nothing but stock apps and FB were installed on it.

Went to Apple to get it replaced and the same thing happened to the replacement phone. Apple replaced it again, and it did the same thing.

Apple ended up giving them an iPhone 4 and it still kept on happening. Once he removed FB, the battery issue disappeared. Years later, we find out about that inaudible audio file playing in the background and was legit the same timeframe of his battery issues.