Here we go again: Ccleaner running in background w/o your permission

[Hmm]

This time it's in quasi-stealth mode. If you check your process explorer, you will see several CCLEANER64.exe files despite not having active monitoring or scheduling enabled. 

[Andavari]

It does have a startup in Scheduled Tasks that it will load during system boot that allows it to avoid producing a UAC prompt, it however should not remain running and I've never noticed it still running in the background. It's a setting located in CCleaner's 'Settings > Advanced' - although I don't know if that's what you're finding.

When enabled as a startup task it's visible in CCleaner itself in 'Tools > Startup > Scheduled Tasks', under the name of: CCleanerSkipUAC

[Clivey]

I think you will find Hmm is correct. 

If you look in Windows Task Scheduler (not through CCleaner), it shows that CCleaner has hidden itself there and yet is not visible when you show the task scheduler within CCleaner. So they are effectively hiding it from you. Not great for a tool that is meant to give you confidence it "cleaning". The Schedule Task has the switch /monitor, so its running in the background, even if Monitoring is turn off.

End Task CCleaner, then Open Windows Task Scheduler and delete it manually from there. Then restart windows.