Noticing a significant increase in data usage since installing iOS9? Wi-fi assist may be the cause. The feature switches your iPhone to cellular usage when it detects you’re in a spot where wi-fi coverage is lacking. For those with unlimited data this wouldn’t be an issue; however, for those who are under a metered data plan this could result with an alarming bill next month. With this setting you could potentially be consuming your data allowance even when you’re just at home or in the office.
It’s a useful feature, but may not be appropriate for all circumstances. The default for this feature is on, but you can switch it off under Settings>Cellular and scroll down to the bottom. Also, you can check out your data usage by using the Reset Statistics feature found below the Wi-Fi- Assist if you want to keep a watchful eye.