[Pox]

Highly unlikely it's an app. It's certainly not jailbroken.

I'm not convinced it's the battery either.

I got my phone at the same time he did (they're both company phones) and he doesn't really do much with it outside of taking pictures and using it for its primary purpose.

[Intar]

If it’s a corporate phone, it could be the MDM and/or MDM policies that are the culprit. Or it could be a pre-loved phone, which companies issue without telling end users.

[Ozark Tom]

Operating system updates never fail to increase the background computing load upon older equipment. This burns through the battery capacity at a higher rate.