Do you use Google Photos? The theory here is that your iPhone spent 9 hours+ uploading/synching photos with Google servers. Google has several apps and services that are managed with Accounts. Google.com. This makes things very difficult to pinpoint what exactly happened for 9 hours. You can check the apps that are running in the background on your iPhone and make an assumption. Were there any Google related website pages open in Safari?
Keep an eye on this and keep narrowing things down until you find out about the cause. NOTE, that this may not happen again for you to continue the research.
Axel F.